If you drive into Dundee from the west along the A90 (T), you will pass on your left a large industrial site. It was formerly occupied by NCR, one of Dundees largest employers, but its factory complex closed some years ago and the site has lain derelict ever since. In 2009 Asda Stores Ltd and MacDonald Estates Group plc, the interveners in the present appeal, applied for planning permission to develop a superstore there. Dundee City Council, the respondents, concluded that a decision to grant planning permission would not be in accordance with the development plan, but was nevertheless justified by other material considerations. Their decision to grant the application is challenged in these proceedings by Tesco Stores Ltd, the appellants, on the basis that the respondents proceeded on a misunderstanding of one of the policies in the development plan: a misunderstanding which, it is argued, vitiated their assessment of whether a departure from the plan was justified. In particular, it is argued that the respondents misunderstood a requirement, in the policies concerned with out of centre retailing, that it must be established that no suitable site is available, in the first instance, within and thereafter on the edge of city, town or district centres. The legislation force at the time of the relevant decision, provides: Section 37(2) of the Town and Country Planning (Scotland) Act 1997, as in In dealing with [an application for planning permission] the authority shall have regard to the provisions of the development plan, so far as material to the application, and to any other material considerations. Section 25 provides: Where, in making any determination under the planning Acts, regard is to be had to the development plan, the determination is, unless material considerations indicate otherwise (a) to be made in accordance with that plan. The development plan The development plan in the present case is an old development plan within the meaning of paragraph 1 of Schedule 1 to the 1997 Act. As such, it is defined by section 24 of the 1997 Act, as that section applied before the coming into force of section 2 of the Planning Etc. (Scotland) Act 2006, as including the approved structure plan and the adopted or approved local plan. The relevant structure plan in the present case is the Dundee and Angus Structure Plan, which became operative in 2002, at a time when the NCR plant remained in operation. As is explained in the introduction to the structure plan, its purpose is to provide a long term vision for the area and to set out the broad land use planning strategy guiding development and change. It includes a number of strategic planning policies. It sets the context for local plans, which translate the strategy into greater detail. Its preparation took account of national planning policy guidelines. The structure plan includes a chapter on town centres and retailing. The introduction explains that the relevant Government guidance is contained in National Planning Policy Guidance 8, Town Centres and Retailing (revised 1998). I note that that document (NPPG 8) was replaced in 2006 by Scottish Planning Policy: Town Centres and Retailing (SPP 8), which was in force at the time of the decision under challenge, and which was itself replaced in 2010 by Scottish Planning Policy (SPP). The relevant sections of all three documents are in generally similar terms. The structure plan continues, at para 5.2: A fundamental principle of NPPG 8 is that of the sequential approach to site selection for new retail developments On this basis, town centres should be the first choice for such developments, followed by edge of centre sites and, only after this, out of centre sites which are currently or potentially accessible by different means of transport. In relation to out of centre developments, that approach is reflected in Town Centres and Retailing Policy 4: Out of Centre Retailing: In keeping with the sequential approach to site selection for new retail developments, proposals for new or expanded out of centre retail developments in excess of 1000 sq m gross will only be acceptable where it can be established that: no suitable site is available, in the first instance, within and thereafter on the edge of city, town or district centres; individually or cumulatively it would not prejudice the vitality and viability of existing city, town or district centres; the proposal would address a deficiency in shopping provision which cannot be met within or on the edge of the above centres; the site is readily accessible by modes of transport other than the car; the proposal is consistent with other Structure Plan policies. The relevant local plan is the Dundee Local Plan, which came into operation in 2005, prior to the closure of the NCR plant. Like the structure plan, it notes that national planning policy guidance emphasises the need to protect and enhance the vitality and viability of town centres. It continues, at para 52.2: As part of this approach planning authorities should adopt a sequential approach to new shopping developments with first preference being town centres, which in Dundees case are the City centre and the District Centres. That approach is reflected in Policy 45: Location of New Retail Developments: The City Centre and District Centres will be the locations of first choice for new or expanded retail developments not already identified in the Local Plan. Proposals for retail developments outwith these locations will only be acceptable where it can be established that: a) no suitable site is available, in the first instance, within and thereafter on the edge of the City Centre or District Centres; and individually or cumulatively it would not prejudice the vitality b) and viability of the City Centre or District Centres; and c) the proposal would address a deficiency in shopping provision which cannot be met within or on the edge of these centres; and d) the site is readily accessible by modes of transport other than the car; and e) It is also relevant to note the guidance given in NPPG 8, as revised in 1998, to which the retailing sections of the structure plan and the local plan referred. Under the heading Sequential Approach, the guidance stated: the proposal is consistent with other Local Plan policies. 12. Planning authorities and developers should adopt a sequential approach to selecting sites for new retail, commercial leisure developments and other key town centre uses First preference should be for town centre sites, where sites or buildings suitable for conversion are available, followed by edge of centre sites, and only then by out of centre sites in locations that are, or can be made easily accessible by a choice of means of transport 13. In support of town centres as the first choice, the Government recognises that the application of the sequential approach requires flexibility and realism from developers and retailers as well as planning authorities. In preparing their proposals developers and retailers should have regard to the format, design, scale of the development, and the amount of car parking in relation to the circumstances of the particular town centre. In addition they should also address the need to identify and assemble sites which can meet not only their requirements, but in a manner sympathetic to the town setting. As part of such an approach, they should consider the scope for accommodating the proposed development in a different built form, and where appropriate adjusting or sub dividing large proposals, in order that their scale might offer a better fit with existing development in the town centre 14. Planning authorities should also be responsive to the needs of retailers and other town centre businesses. In consultation with the private sector, they should assist in identifying sites in the town centre which could be suitable and viable, for example, in terms of size and siting for the proposed use, and are likely to become available in a reasonable time 15. Only if it can be demonstrated that all town centre options have been thoroughly addressed and a view taken on availability, should less central sites in out of centre locations be considered for key town centre uses. Where development proposals in such locations fall outwith the development plan framework, it is for developers to demonstrate that town centre and edge of centre options have been thoroughly assessed. Even where a developer, as part of a sequential approach, demonstrates an out of centre location to be the most appropriate, the impact on the vitality and viability of existing centres still has to be shown to be acceptable The consideration of the application The interveners application was for planning permission to develop a foodstore, caf and petrol filling station, with associated car parking, landscaping and infrastructure, including access roads. The proposals also involved improvements to the junction with the A90 (T), the upgrading of a pedestrian underpass, the provision of footpaths and cycle ways, and improvements to adjacent roadways. A significant proportion of the former NCR site lay outside the application site. It was envisaged that vehicular access to this land could be achieved using one of the proposed access roads. In his report to the respondents, the Director of City Development advised that the application was contrary to certain aspects of the employment and retailing policies of the development plan. In relation to the employment policies, in particular, the proposal was contrary to policies which required the respondents to safeguard the NCR site for business use. The Director considered however that the application site was unlikely to be re developed for business uses in the short term, and that its re development as proposed would improve the development prospects of the remainder of the NCR site. In addition, the infrastructure improvements would provide improved access which would benefit all businesses in an adjacent industrial estate. In relation to the retailing policies, the Director considered the application in the light of the criteria in Retailing Policy 4 of the structure plan. In relation to the first criterion he stated: It must be demonstrated, in the first instance, that no suitable site is available for the development either within the city/district centres or, thereafter on the edge of these centres While noting that the Lochee District Centre lies within the primary catchment area for the proposal, [the retail statement submitted on behalf of the interveners] examines the potential site opportunities in and on the edge of that centre and also at the Hilltown and Perth Road District Centres. The applicants conclude that there are no sites or premises available in or on the edge of existing centres capable of accommodating the development under consideration. Taking account of the applicants argument it is accepted that at present there is no suitable site available to accommodate the proposed development. In relation to the remaining criteria, the Director concluded that the proposed development was likely to have a detrimental effect on the vitality and viability of Lochee District Centre, and was therefore in conflict with the second criterion. The potential impact on Lochee could however be minimised by attaching conditions to any permission granted so as to restrict the size of the store, limit the type of goods for sale and prohibit the provision of concessionary units. The proposal was also considered to be in conflict with the third criterion: there was no deficiency in shopping provision which the proposal would address. The fourth criterion, concerned with accessibility by modes of transport other than the car, was considered to be met. Similar conclusions were reached in relation to the corresponding criteria in Policy 45 of the local plan. In view of the conflict with the employment and retailing policies, the Director considered that the proposal did not fully comply with the provisions of the development plan. He identified however two other material considerations of particular significance. First, the proposed development would bring economic benefits to the city. The closure of the NCR factory had been a major blow to the economy, but the re development of the application site would create more jobs than had been lost when the factory finally closed. The creation of additional employment opportunities within the city was considered to be a strong material consideration. Secondly, the development would also provide a number of planning benefits. There would be improvements to the strategic road network which would assist in the free flow of traffic along the A90 (T). The development would also assist in the re development of the whole of the former NCR site through the provision of enhanced road access and the clearance of buildings from the site. The access improvements would also assist in the development of an economic development area to the west. These benefits were considered to be another strong material consideration. The Director concluded that the proposal was not in accordance with the development plan, particularly with regard to the employment and retailing policies. There were however other material considerations of sufficient weight to justify setting aside those policies and offering support for the development, subject to suitable conditions. He accordingly recommended that consent should be granted, subject to specified conditions. The application was considered by the respondents entire council sitting as the respondents Development Quality Committee. After hearing submissions on behalf of the interveners and also on behalf of the appellants, the respondents decided to follow the Directors recommendation. The reasons which they gave for their decision repeated the Directors conclusions: It is concluded that the proposal does not undermine the core land use and environmental strategies of the development plan. The planning and economic benefits that would accrue from the proposed development would be important to the future development and viability of the city as a regional centre. These benefits are considered to be of a significant weight and sufficient to set aside the relevant provisions of the development plan. The present proceedings The submissions on behalf of the appellants focused primarily upon an alleged error of interpretation of the first criterion in Retailing Policy 4 of the structure plan, and of the equivalent criterion in Policy 45 of the local plan. If there was a dispute about the meaning of a development plan policy which the planning authority was bound to take into account, it was for the court to determine what the words were capable of meaning. If the planning authority attached a meaning to the words which they were not properly capable of bearing, then it made an error of law, and failed properly to understand the policy. In the present case, the Director had interpreted suitable as meaning suitable for the development proposed by the applicant; and the respondents had proceeded on the same basis. That was not however a tenable meaning. Properly interpreted, suitable meant suitable for meeting identified deficiencies in retail provision in the area. Since no such deficiency had been identified, it followed on a proper interpretation of the plan that the first criterion did not require to be considered: it was inappropriate to undertake the sequential approach. The Directors report had however implied that the first criterion was satisfied, and that the proposal was to that extent in conformity with the sequential approach. The respondents had proceeded on that erroneous basis. They had thus failed to identify correctly the extent of the conflict between the proposal and the development plan. In consequence, their assessment of whether other material considerations justified a departure from the plan was inherently flawed. The respondents had compounded their error, it was submitted, by treating the proposed development as definitive when assessing whether a suitable site was available. That approach permitted developers to drive a coach and horses through the sequential approach: they could render the policy nugatory by the simple expedient of putting forward proposals which were so large that they could only be accommodated outside town and district centres. In the present case, there was a site available in Lochee which was suitable for food retailing and which was sequentially preferable to the application site. The Lochee site had been considered as part of the assessment of the proposal, but had been found to be unsuitable because it could not accommodate the scale of development to which the interveners aspired. In response, counsel for the respondents submitted that it was for the planning authority to interpret the relevant policy, exercising its planning judgment. Counsel accepted that, if there was a dispute about the meaning of the words in a policy document, it was for the court to determine as a matter of law what the words were capable of meaning. The planning authority would only make an error of law if it attached a meaning to the words which they were not capable of bearing. In the present case, the relevant policies required all the specified criteria to be satisfied. The respondents had proceeded on the basis that the proposal failed to accord with the second and third criteria. In those circumstances, the respondents had correctly concluded that the proposal was contrary to the policies in question. How the proposal had been assessed against the first criterion was immaterial. So far as concerned the assessment of suitable sites, the interveners retail statement reflected a degree of flexibility. There had been a consideration of all sites of at least 2.5 ha, whereas the application site extended to 6.68 ha. The interveners had also examined sites which could accommodate only food retailing, whereas their application had been for both food and non food retailing. The Lochee site extended to only 1.45 ha, and could accommodate a store of only half the size proposed. It also had inadequate car parking. The Director, and the respondents, had accepted that it was not a suitable site for these reasons. Discussion It has long been established that a planning authority must proceed upon a proper understanding of the development plan: see, for example, Gransden & Co Ltd v Secretary of State for the Environment (1985) 54 P & CR 86, 94 per Woolf J, affd (1986) 54 P & CR 361; Horsham DC v Secretary of State for the Environment (1991) 63 P & CR 219, 225 226 per Nolan LJ. The need for a proper understanding follows, in the first place, from the fact that the planning authority is required by statute to have regard to the provisions of the development plan: it cannot have regard to the provisions of the plan if it fails to understand them. It also follows from the legal status given to the development plan by section 25 of the 1997 Act. The effect of the predecessor of section 25, namely section 18A of the Town and Country (Planning) Scotland Act 1972 (as inserted by section 58 of the Planning and Compensation Act 1991), was considered by the House of Lords in the case of City of Edinburgh Council v Secretary of State for Scotland 1998 SC (HL) 33, [1997] 1 WLR 1447. It is sufficient for present purposes to cite a passage from the speech of Lord Clyde, with which the other members of the House expressed their agreement. At p 44, 1459, his Lordship observed: In the practical application of sec 18A it will obviously be necessary for the decision maker to consider the development plan, identify any provisions in it which are relevant to the question before him and make a proper interpretation of them. His decision will be open to challenge if he fails to have regard to a policy in the development plan which is relevant to the application or fails properly to interpret it. In the present case, the planning authority was required by section 25 to consider whether the proposed development was in accordance with the development plan and, if not, whether material considerations justified departing from the plan. In order to carry out that exercise, the planning authority required to proceed on the basis of what Lord Clyde described as a proper interpretation of the relevant provisions of the plan. We were however referred by counsel to a number of judicial dicta which were said to support the proposition that the meaning of the development plan was a matter to be determined by the planning authority: the court, it was submitted, had no role in determining the meaning of the plan unless the view taken by the planning authority could be characterised as perverse or irrational. That submission, if correct, would deprive sections 25 and 37(2) of the 1997 Act of much of their effect, and would drain the need for a proper interpretation of the plan of much of its meaning and purpose. It would also make little practical sense. The development plan is a carefully drafted and considered statement of policy, published in order to inform the public of the approach which will be followed by planning authorities in decision making unless there is good reason to depart from it. It is intended to guide the behaviour of developers and planning authorities. As in other areas of administrative law, the policies which it sets out are designed to secure consistency and direction in the exercise of discretionary powers, while allowing a measure of flexibility to be retained. Those considerations point away from the view that the meaning of the plan is in principle a matter which each planning authority is entitled to determine from time to time as it pleases, within the limits of rationality. On the contrary, these considerations suggest that in principle, in this area of public administration as in others (as discussed, for example, in R (Raissi) v Secretary of State for the Home Department [2008] QB 836), policy statements should be interpreted objectively in accordance with the language used, read as always in its proper context. That is not to say that such statements should be construed as if they were statutory or contractual provisions. Although a development plan has a legal status and legal effects, it is not analogous in its nature or purpose to a statute or a contract. As has often been observed, development plans are full of broad statements of policy, many of which may be mutually irreconcilable, so that in a particular case one must give way to another. In addition, many of the provisions of development plans are framed in language whose application to a given set of facts requires the exercise of judgment. Such matters fall within the jurisdiction of planning authorities, and their exercise of their judgment can only be challenged on the ground that it is irrational or perverse (Tesco Stores Ltd v Secretary of State for the Environment [1995] 1 WLR 759, 780 per Lord Hoffmann). Nevertheless, planning authorities do not live in the world of Humpty Dumpty: they cannot make the development plan mean whatever they would like it to mean. The principal authority referred to in relation to this matter was the judgment of Brooke LJ in R v Derbyshire County Council, Ex p Woods [1997] JPL 958 at 967. Properly understood, however, what was said there is not inconsistent with the approach which I have described. In the passage in question, Brooke LJ stated: If there is a dispute about the meaning of the words included in a policy document which a planning authority is bound to take into account, it is of course for the court to determine as a matter of law what the words are capable of meaning. If the decision maker attaches a meaning to the words they are not properly capable of bearing, then it will have made an error of law, and it will have failed properly to understand the policy. By way of illustration, Brooke LJ referred to the earlier case of Northavon DC v Secretary of State for the Environment [1993] JPL 761, which concerned a policy applicable to institutions standing in extensive grounds. As was observed, the words spoke for themselves, but their application to particular factual situations would often be a matter of judgment for the planning authority. That exercise of judgment would only be susceptible to review in the event that it was unreasonable. The latter case might be contrasted with the case of R (Heath and Hampstead Society) v Camden LBC [2008] 2 P & CR 233, where a planning authoritys decision that a replacement dwelling was not materially larger than its predecessor, within the meaning of a policy, was vitiated by its failure to understand the policy correctly: read in its context, the phrase materially larger referred to the size of the new building compared with its predecessor, rather than requiring a broader comparison of their relative impact, as the planning authority had supposed. Similarly in City of Edinburgh Council v Scottish Ministers 2001 SC 957 the reporters decision that a licensed restaurant constituted similar licensed premises to a public house, within the meaning of a policy, was vitiated by her misunderstanding of the policy: the context was one in which a distinction was drawn between public houses, wine bars and the like, on the one hand, and restaurants, on the other. A provision in the development plan which requires an assessment of whether a site is suitable for a particular purpose calls for judgment in its application. But the question whether such a provision is concerned with suitability for one purpose or another is not a question of planning judgment: it is a question of textual interpretation, which can only be answered by construing the language used in its context. In the present case, in particular, the question whether the word suitable, in the policies in question, means suitable for the development proposed by the applicant, or suitable for meeting identified deficiencies in retail provision in the area, is not a question which can be answered by the exercise of planning judgment: it is a logically prior question as to the issue to which planning judgment requires to be directed. It is of course true, as counsel for the respondents submitted, that a planning authority might misconstrue part of a policy but nevertheless reach the same conclusion, on the question whether the proposal was in accordance with the policy, as it would have reached if it had construed the policy correctly. That is not however a complete answer to a challenge to the planning authoritys decision. An error in relation to one part of a policy might affect the overall conclusion as to whether a proposal was in accordance with the development plan even if the question whether the proposal was in conformity with the policy would have been answered in the same way. The policy criteria with which the proposal was considered to be incompatible might, for example, be of less weight than the criteria which were mistakenly thought to be fulfilled. Equally, a planning authority might misconstrue part of a policy but nevertheless reach the same conclusion as it would otherwise have reached on the question whether the proposal was in accordance with the development plan. Again, however, that is not a complete answer. Where it is concluded that the proposal is not in accordance with the development plan, it is necessary to understand the nature and extent of the departure from the plan which the grant of consent would involve in order to consider on a proper basis whether such a departure is justified by other material considerations. In the present case, the Lord Ordinary rejected the appellants submissions on the basis that the interpretation of planning policy was always primarily a matter for the planning authority, whose assessment could be challenged only on the basis of unreasonableness: there was, in particular, more than one way in which the sequential approach could reasonably be applied ([2010] CSOH 128, para 23). For the reasons I have explained, that approach does not correctly reflect the role which the court has to play in the determination of the meaning of the development plan. A different approach was adopted by the Second Division: since, it was said, the proposal was in head on conflict with the retail and employment policies of the development plan, and the sequential approach offered no justification for it, a challenge based upon an alleged misapplication of the sequential approach was entirely beside the point (2011 SC 457, [2011] CSIH 9, para 38). For the reasons I have explained, however, even where a proposal is plainly in breach of policy and contrary to the development plan, a failure properly to understand the policy in question may result in a failure to appreciate the full extent or significance of the departure from the development plan which the grant of consent would involve, and may consequently vitiate the planning authoritys determination. Whether there has in fact been a misunderstanding of the policy, and whether any such misunderstanding may have led to a flawed decision, has therefore to be considered. I turn then to the question whether the respondents misconstrued the policies in question in the present case. As I have explained, the appellants primary contention is that the word suitable, in the first criterion of Retailing Policy 4 of the structure plan and the corresponding Policy 45 of the local plan, means suitable for meeting identified deficiencies in retail provision in the area, whereas the respondents proceeded on the basis of the construction placed upon the word by the Director of City Development, namely suitable for the development proposed by the applicant. I accept, subject to a qualification which I shall shortly explain, that the Director and the respondents proceeded on the latter basis. Subject to that qualification, it appears to me that they were correct to do so, for the following reasons. First, that interpretation appears to me to be the natural reading of the policies in question. They have been set out in paras 4 and 5 above. Read short, Retailing Policy 4 of the structure plan states that proposals for new or expanded out of centre retail developments will only be acceptable where it can be established that a number of criteria are satisfied, the first of which is that no suitable site is available in a sequentially preferable location. Policy 45 of the local plan is expressed in slightly different language, but it was not suggested that the differences were of any significance in the present context. The natural reading of each policy is that the word suitable, in the first criterion, refers to the suitability of sites for the proposed development: it is the proposed development which will only be acceptable at an out of centre location if no suitable site is available more centrally. That first reason for accepting the respondents interpretation of the policy does not permit of further elaboration. Secondly, the interpretation favoured by the appellants appears to me to conflate the first and third criteria of the policies in question. The first criterion concerns the availability of a suitable site in a sequentially preferable location. The third criterion is that the proposal would address a deficiency in shopping provision which cannot be met in a sequentially preferable location. If suitable meant suitable for meeting identified deficiencies in retail provision, as the appellants contend, then there would be no distinction between those two criteria, and no purpose in their both being included. Thirdly, since it is apparent from the structure and local plans that the policies in question were intended to implement the guidance given in NPPG 8 in relation to the sequential approach, that guidance forms part of the relevant context to which regard can be had when interpreting the policies. The material parts of the guidance are set out in para 6 above. They provide further support for the respondents interpretation of the policies. Paragraph 13 refers to the need to identify sites which can meet the requirements of developers and retailers, and to the scope for accommodating the proposed development. Paragraph 14 advises planning authorities to assist the private sector in identifying sites which could be suitable for the proposed use. Throughout the relevant section of the guidance, the focus is upon the availability of sites which might accommodate the proposed development and the requirements of the developer, rather than upon addressing an identified deficiency in shopping provision. The latter is of course also relevant to retailing policy, but it is not the issue with which the specific question of the suitability of sites is concerned. I said earlier that it was necessary to qualify the statement that the Director and the respondents proceeded, and were correct to proceed, on the basis that suitable meant suitable for the development proposed by the applicant. As paragraph 13 of NPPG 8 makes clear, the application of the sequential approach requires flexibility and realism from developers and retailers as well as planning authorities. The need for flexibility and realism reflects an inbuilt difficulty about the sequential approach. On the one hand, the policy could be defeated by developers and retailers taking an inflexible approach to their requirements. On the other hand, as Sedley J remarked in R v Teesside Development Corporation, Ex p William Morrison Supermarket plc and Redcar and Cleveland BC [1998] JPL 23, 43, to refuse an out of centre planning consent on the ground that an admittedly smaller site is available within the town centre may be to take an entirely inappropriate business decision on behalf of the developer. The guidance seeks to address this problem. It advises that developers and retailers should have regard to the circumstances of the particular town centre when preparing their proposals, as regards the format, design and scale of the development. As part of such an approach, they are expected to consider the scope for accommodating the proposed development in a different built form, and where appropriate adjusting or sub dividing large proposals, in order that their scale may fit better with existing development in the town centre. The guidance also advises that planning authorities should be responsive to the needs of retailers. Where development proposals in out of centre locations fall outside the development plan framework, developers are expected to demonstrate that town centre and edge of centre options have been thoroughly assessed. That advice is not repeated in the structure plan or the local plan, but the same approach must be implicit: otherwise, the policies would in practice be inoperable. It follows from the foregoing that it would be an over simplification to say that the characteristics of the proposed development, such as its scale, are necessarily definitive for the purposes of the sequential test. That statement has to be qualified to the extent that the applicant is expected to have prepared his proposals in accordance with the recommended approach: he is, for example, expected to have had regard to the circumstances of the particular town centre, to have given consideration to the scope for accommodating the development in a different form, and to have thoroughly assessed sequentially preferable locations on that footing. Provided the applicant has done so, however, the question remains, as Lord Glennie observed in Lidl UK GmbH v Scottish Ministers [2006] CSOH 165, para 14, whether an alternative site is suitable for the proposed development, not whether the proposed development can be altered or reduced so that it can be made to fit an alternative site. In the present case, it is apparent that a flexible approach was adopted. The interveners did not confine their assessment to sites which could accommodate the development in the precise form in which it had been designed, but examined sites which could accommodate a smaller development and a more restricted range of retailing. Even taking that approach, however, they did not regard the Lochee site vacated by the appellants as being suitable for their needs: it was far smaller than they required, and its car parking facilities were inadequate. In accepting that assessment, the respondents exercised their judgment as to how the policy should be applied to the facts: they did not proceed on an erroneous understanding of the policy. Finally, I would observe that an error by the respondents in interpreting their policies would be material only if there was a real possibility that their determination might otherwise have been different. In the particular circumstances of the present case, I am not persuaded that there was any such possibility. The considerations in favour of the proposed development were very powerful. They were also specific to the particular development proposed: on the information before the respondents, there was no prospect of any other development of the application site, or of any development elsewhere which could deliver equivalent planning and economic benefits. Against that background, the argument that a different decision might have been taken if the respondents had been advised that the first criterion in the policies in question did not arise, rather than that criterion had been met, appears to me to be implausible. Conclusion For these reasons, and those given by Lord Hope, with which I am in entire agreement, I would dismiss the appeal. LORD HOPE The question that lies at the heart of this case is whether the respondents acted unlawfully in their interpretation of the sequential approach which both the structure plan and the relevant local plan required them to adopt to new retail developments within their area. According to that approach, proposals for new or expanded out of centre developments of this kind are acceptable only where it can be established, among other things, that no suitable site is available, in the first instance, within and thereafter on the edge of city, town or district centres. Is the test as to whether no suitable site is available in these locations, when looked at sequentially, to be addressed by asking whether there is a site in each of them in turn which is suitable for the proposed development? Or does it direct attention to the question whether the proposed development could be altered or reduced so as to fit into a site which is available there as a location for this kind of development? The sequential approach is described in National Planning Policy Guidance Policy 8, Town Centres and Retailing, para 5.2 as a fundamental principle of NPPG 8. In R v Rochdale Metropolitan Borough Council, Ex p Milne, 31 July 2000, not reported, paras 48 49, Sullivan J said that it was not unusual for development plan polices to pull in different directions and, having regard to what Lord Clyde said about the practical application of the statutory rule in City of Edinburgh v Secretary of State for Scotland 1998 SC (HL) 33 at p 44, that he regarded as untenable the proposition that if there was a breach of any one policy in a development plan a proposed development could not be said to be in accordance with the plan. In para 52 he said that the relative importance of a given policy to the overall objectives of the development plan was essentially a matter for the judgment of the local planning authority and that a legalistic approach to the interpretation of development plan policies was to be avoided. I see no reason to question these propositions, to which Mr Kingston QC for the appellants drew our attention in his reply to Mr Armstrongs submissions for the respondents. But I do not think that they are in point in this case. We are concerned here with a particular provision in the planning documents to which the respondents are required to have regard by the statute. The meaning to be given to the crucial phrase is not a matter that can be left to the judgment of the planning authority. Nor, as the Lord Ordinary put it in his opinion at [2010] CSOH 128, para 23, is the interpretation of the policy which it sets out primarily a matter for the decision maker. As Mr Thomson for the interveners pointed out, the challenge to the respondents decision to follow the Directors recommendation and approve the proposed development is not that it was Wednesbury unreasonable but that it was unlawful. I agree with Lord Reed that the issue is one of law, reading the words used objectively in their proper context. In Lidl UK GmbH v The Scottish Ministers [2006] CSOH 165 the appellants appealed against a decision of the Scottish Ministers to refuse planning permission for a retail unit to be developed on a site outwith Irvine town centre. The relevant provision in the local plan required the sequential approach to be adopted to proposals for new retail development out with the town centre boundaries. Among the criteria that had to be satisfied was the requirement that no suitable sites were available, or could reasonably be made available, in or on the edge of existing town centres. In other words, town centre sites were to be considered first before edge of centre or out of town sites. The reporter held that the existing but soon to be vacated Lidl town centre site was suitable for the proposed development, although it was clear as a matter of fact that this site could not accommodate it. In para 13 Lord Glennie noted that counsel for the Scottish Ministers accepted that a site would be suitable in terms of the policy only if it was suitable for, or could accommodate, the development as proposed by the developer. In para 14 he said that the question was whether the alternative town centre site was suitable for the proposed development, not whether the proposed development could be altered or reduced so that it could fit in to it. Mr Kingston submitted that Lord Glennies approach would rob the sequential approach of all its force, and in the Inner House it was submitted that his decision proceeded on a concession by counsel which ought not to have been made: [2011] CSIH 9, 2011 SC 457, para 31. But I think that Lord Glennies interpretation of the phrase was sound and that counsel was right to accept that it had the meaning which she was prepared to give to it. The wording of the relevant provision in the local plan in that case differed slightly from that with which we are concerned in this case, as it included the phrase or can reasonably be made available. But the question to which it directs attention is the same. It is the proposal for which the developer seeks permission that has to be considered when the question is asked whether no suitable site is available within or on the edge of the town centre. The context in which the word suitable appears supports this interpretation. It is identified by the opening words of the policy, which refer to proposals for new or expanded out of centre retail developments and then set out the only circumstances in which developments outwith the specified locations will be acceptable. The words the proposal which appear in the third and fifth of the list of the criteria which must be satisfied serve to reinforce the point that the whole exercise is directed to what the developer is proposing, not some other proposal which the planning authority might seek to substitute for it which is for something less than that sought by the developer. It is worth noting too that the phrase no suitable site is available appears in Policy 46 of the local plan relating to commercial developments. Here too the context indicates that the issue of suitability is directed to the developers proposals, not some alternative scheme which might be suggested by the planning authority. I do not think that this is in the least surprising, as developments of this kind are generated by the developers assessment of the market that he seeks to serve. If they do not meet the sequential approach criteria, bearing in mind the need for flexibility and realism to which Lord Reed refers in para 28, above, they will be rejected. But these criteria are designed for use in the real world in which developers wish to operate, not some artificial world in which they have no interest doing so. For these reasons which I add merely as a footnote I agree with Lord Reed, for all the reasons he gives, that this appeal should be dismissed. I would affirm the Second Divisions interlocutor.