## Principle 6: Develop A Relevant And Robust Evidence Base For Housing And Other Topics

Guide Questions 
- 
Is the evidence proportionate and relevant to the plan's scope? 
- 
How is the evidence base being used to justify the plan's approach and content? 
- 
How is evidence being used to address specific issues such as infrastructure? 
- 
What is the scope for commissioning joint studies with neighbouring authorities? 6.1 
The National Planning Policy Framework (NPPF) makes it clear that evidence, or 'objectively assessed development needs' should underpin the overall strategy and each policy in the local plan. When examining plans the Planning Inspectorate will ask a series of key questions including: is the content of the local plan document justified by the evidence? Part of their 
assessment of the soundness of the plan will include considering the source(s) of the evidence and how up to date it is.  
 
6.2 
Evidence should inform what is in the plan, rather than being collected 
retrospectively in an attempt to justify the plan. You should have a clear idea 
about what the evidence is needed for, how it is going to be used and how much detail is needed. The general approach is that it is the quality, not the quantity, of the evidence that counts.  
 
6.3 
Certain evidence base studies are still required by the NPPF, although the main thing to remember is proportionality.  Lengthy, time-consuming and 
expensive evidence collecting is the major cause of delay in getting a plan finished.  To help manage this issue, the following questions are helpful. 
- Is it really needed to inform plan choices or the development of option (the 
'so what' test)? 
- What would happen if it didn't exist? 
- How relevant is it to the plan area and objectives? 
- If it is relevant, what is the best way of getting it? 
- Can existing evidence (including the baseline within the Sustainability 
Appraisal) be drawn upon, subject to checking its relevance and currency? 6.4 
Many councils consider their Strategic Housing Land Availability Assessment, Strategic Housing Market Assessment and Employment Land 
Review to be key evidence.  Make sure that these are as up to date and 
relevant as possible when the plan is examined. However, what should constitute the evidence base is a local matter; for example if water supply and sewage treatment issues are important then water cycle studies and evidence from the Environment Agency and utility companies would be relevant to support the plan's approach. There is no set list of relevant evidence; it will depend on the issues that the plan seeks to address and on local circumstances. 

 6.5 
Once you've decided what evidence you need, think about the different ways 
of getting it. What already exists and is easily available? What do you need to get from external stakeholders? What could be prepared jointly with others? And what could be assembled by other organisations? The main options are summarised below. 
 
- **In-house**: from your own data, data held elsewhere in the council, from 
other strategies or research work. 
- **Consultants**: where the expertise or capacity doesn't exist in house or 
time is tight. 
- **External stakeholders:** the development industry, health providers, utility 
and infrastructure providers, national agencies. 
- **Use local knowledge:** Outputs from community engagement exercises 
can provide useful evidence (for example, by canvassing opinion on perceived shortfalls in community facilities). Dividing the plan area up into easily identifiable character areas, and providing simple profiles on each, is a good way to start the dialogue with the local community and to focus your thoughts. Neighbourhoods or parish/town councils can have a particularly helpful role in contributing to area profiles and assessments local infrastructure needs.  
- **Monitoring:** Annual monitoring reports and evidence of the impact of your 
policies will be part of the database that helps inform your choices. For this to work, make sure that the range of indicators used when monitoring genuinely helps you to understand plan implementation and are consistent over time to enable you to build up a profile of change. 
- **Sustainability appraisal:** The sustainability appraisal should be an 
integral part of the evidence base and publishing its findings at each stage 
in the plan preparation process will help demonstrate that the it has played a full and positive role in the development of the local plan. 6.6 
Working with others such as neighbouring authorities to produce evidence could save you money and resources, and ensure consistency across a subregion. Other authorities may have more recent data or be willing to share 
the cost of assembling or monitoring data. Joint studies could be commissioned even if you are at different stages of plan preparation if the study will still be considered up to date at the time of your examination. Paragraphs 178-182 of the NPPF set out the expectations for such joint working centred on fulfilling the duty to co-operate and a shared evidence base. Strategic housing market and flood risk assessments in particular, lend themselves to joint working. 
6.7 
Before commencing or commissioning work, carefully consider the terms of reference of any studies and ensure that the outputs provide the required information that will help development options. As far as possible, specify the 
same time horizons for topic-based studies so that forecasts, projections and 
associated land and premises requirements relate to the same time horizons 
within all evidence base documents (and the local plan being prepared). Consider too the need for spatially disaggregated data. For example, while 
district or borough level data might provide useful context, they may not be pivotal in terms of generating and testing options. 
6.8 
Ensure that the level of evidence collected reflects the level of detail in the plan. For example, if the local plan is set to contain certain strategic sites, 
then information on the characteristics of these sites and their relationships 
with neighbouring areas will be necessary to facilitate a robust appraisal. 6.9 
The NPPF is clear that the plan must take demographic considerations including migration trends into account.  Neither a capacity based approach nor one based on past building rates is appropriate. The impact of 
anticipated employment levels should be taken into account but only as one factor within the context of the overall demographic profile of the area. 6.10 
Housing need assessments (Strategic Housing Market Assessment) should 
cover the housing market area which is likely to be cross-boundary rather 
than just the authority area. You should clearly explain the methodology. The 
assessments of need and housing land availability must be done as separate exercises to ensure their objectivity. 6.11 
The strategic housing land availability assessment (SHLAA) should cover all potential sources of supply.  The SHLAA should be prepared in consultation with the development industry and key stakeholders, and should identify as many potential housing sites in the area as possible.  Agreeing a 
methodology with neighbouring authorities will result in a consistent approach across the housing market area.  The local plan should then seek 
to decide which of these identified sites should be allocated for housing. 6.12 
The SHLAA forms part of the evidence base and is not examined in its own right, but it does need to be sufficiently robust to justify and support the delivery of the plan.  A plan is not unsound just because there is no SHLAA in place, but any available evidence of housing land supply should be updated taking into account the same basic principles.  Housing supply 
evidence needs to be up to date, convincing and in accordance with NPPF paragraphs 47, 50 and 159. 6.13 
The local plan should contain a housing trajectory, or provide a Housing Implementation Strategy in the evidence base (sometimes this is combined 
with an Infrastructure Delivery Strategy.  This is used to demonstrate 
housing delivery against housing need.  Some authorities are seeking to "backload" their housing trajectories citing the poor housing market as a reason for doing so.  However, the point about the trajectory is that it should 
relate to housing need.  There is little point in planning to provide additional 
housing in the latter part of the plan period if the need is for housing in the early plan period.  Thus to be effective housing policies need to be closely related to where and when the housing need is anticipated. 

 6.14 
The NPPF (paragraph 48) says that authorities may make an allowance for 
windfall sites (excluding residential gardens) in the 5-year housing land 
supply, if there is compelling evidence that they have been consistently available and will continue to be a reliable source of supply. Windfall sites' 
are sites that have not been identified in the plan preparation process and hence unexpectedly become available.  They are unforeseen and unplanned-for sites, so a site identified in a SHLAA cannot be a 'windfall site' even if it has not been chosen as one of the allocated sites. 
 
6.15 
In trying to determine whether there has been a record of 'persistent under delivery' that will impact future plan-requirements (NPPF, paragraph 47), take a sensible and pragmatic view.  Look at a period that is long enough to take into account the peaks and troughs of the property cycle. 6.16 
If your plan proposes over-provision (for example, as part of a growth 
strategy) this should also be shown to be justified by the evidence and, crucially, deliverable.  However, if a figure is proposed in the local plan that is lower than the assessed need the justification for doing this must be very strong and supported by evidence on the relevant constraints.  Any requirement to provide housing in an adjoining authority should be demonstrated agreement between the authorities. Keeping evidence up to date 
 
6.17 
You may need to refresh the evidence base as the plan develops, as new 
sources become available, some become out of date, new issues arise to be addressed.  Still aim to keep it relevant and proportionate, and get what you can from other plans and strategies.  Document your evidence as it evolves.  
Review the evidence for timeliness (is it up-to-date/the latest available), relevance and completeness in terms of coverage of the plan area. 
6.18 
Evidence can be challenged if it is too old.  Some issues (for example, 
landscape character) are less transient and liable to short-term change than others (for example, housing markets).  Evidence base documents relating to retail, employment and housing that were completed three or more years before the submission date are particularly at risk of having been overtaken 
by events.  This because the evidence used in these documents goes back some way prior to it being finalised. With this in mind, identify the aspects of the evidence base which only need to be established once during the local plan process (for example, biodiversity and geo-diversity status) and those 
which may need to be periodically revisited. 
6.19 
For evidence that needs to be revisited, arrangements should be put in place to ensure that information is updated as appropriate.  Sometimes, as you go 
through the plan- making process, new evidence might be needed to 
address a new issue, or to strengthen an area where there is little existing evidence to support the plan.  Your evidence will also have uses elsewhere within the council and could be one of the main areas where you help neighbourhoods or parishes with neighbourhood planning. 

 

6.20 
Rather than treat evidence base production as a one-off, some councils are 
treating the understanding and sharing of evidence as an on-going process.  
The frequency and resources needed are dependent on the importance of the dataset to the plan.  Light-touch reviews can demonstrate whether it is time for a more fundamental reappraisal (for example if the latest population and household formation information is significantly different from the original assumptions made). 
 

## Sharing And Presentation Of Evidence

 

6.21 
The local plan must not contain assertions of fact not supported by the evidence where evidence has been prepared by consultants or other departments.  Officers need to make sure they understand it fully if they are to rely on it. 
6.22 
The evidence base should be set out clearly; this includes making sure that evidence is as easy to read as possible and that the evidence library/webpage is well-structured. Where appropriate, produce brief topic papers on particular subjects so that all the evidence is easily accessible in 
one place (this can also be useful in highlighting any gaps).  Use signposting via footnotes, and make sure any conflicts within the evidence base are explained.  Provide executive summaries or non-technical summaries of particularly large or technical pieces of evidence; this is a legal requirement for the Sustainability Appraisal. 6.23 
Finally, put arrangements in place for sharing the information with your councillors and local community.  Use it effectively to help councillors with 
decision making and for the local community to see clear links between the 
evidence and the emerging local plan.  
 

## Further Information

Plan making direct support - review of your evidence base Objectively Assessed Need and Housing Targets: Technical Advice Note More PAS guidance on approaching your objectively assessed needs is available in '10 key principles for owning your housing number'  
 
Q: When does existing evidence become out of date? 

 
A: Monitoring will provide some indication.  Any changes in national policy should be reflected in evidence, where necessary.  A common sense approach should always apply.  If evidence was based on particular circumstances which no longer prevail, then it is likely to be out of date.  The age of evidence is not, in itself, the only determining factor. 

Q: Is there any guidance available on how to calculate the 5-year land supply? 

 
A: The Planning Practice Guidance covers this here: http://planningguidance.planningportal.gov.uk/blog/guidance/housing-and-economicland-availability-assessment/stage-5-final-evidence-base/ 
Q: One key issue is about factoring in unmet need - which is preferred: 
'Sedgefield' or 'Liverpool' method? A: The Liverpool and Sedgefield approaches have both been highlighted as good methods for calculating historic under supply in their (now abolished) regional spatial strategies. The Sedgefield method of calculating land supply involves adding any shortfall of housing in the local plan from previous years over the next five years of the plan period, whereas the Liverpool method spreads the shortfall over the whole remaining plan period. The Planning Practice Guidance covers past performance in meeting housing need: 
http://planningguidance.planningportal.gov.uk/blog/guidance/housing-and-economicland-availability-assessment/stage-5-final-evidence-base/ 
Q: Can you use more than just sites with permission in your five year housing supply? A: The Planning Practice Guidance covers this issue in this section: http://planningguidance.planningportal.gov.uk/blog/guidance/housing-and-economicland-availability-assessment/stage-5-final-evidence-base/ You are likely to have problems if you submit a plan that doesn't provide for a fiveyear land supply. You will need to provide robust evidence to support any assumptions about including (or excluding) sites from your calculation. Let's say you take this year as 'Year Five'; go back five years and see how many houses that have been built in the last 5 years, had permission at the start of that period.  So you can see the kind of amount that comes forward in a five year period in your council.  This may be quite low recently, but should still help to illustrate the point that in any five year period, you can expect 'x' to come forward on sites not currently benefitting from permission. In addition to sites under construction and with planning permission you may also consider: 
 

- sites scheduled to go to committee where there is a recommendation to 
approve 
- sites at advanced pre-app (or really at any stage of pre-app provided they are 
not large and 'complex') - where there is a likelihood they will get PP in the next 12-18 months 
- sites in the SHLAA or allocated sites that you have had conversations about 
contributing early, and have robust evidence on delivery rates 
- windfall sites - not in year one, as these are likely to already have permission 
if they are contributing to year one.   Exercise caution on windfalls, mainly because, if you do a 'robust' SHLAA, then you will have identified all available 
land (within reason) so once it is in the SHLAA it can't be windfall. 
 This is not intended to be an exhaustive, or 'fool proof' list.  Evidence is key to all of your assumptions about sites coming forward that do not currently have planning permission.  There is also a risk of double-counting.  Ensure that any sites in the pipeline are not also counted as sites you are counting from the SHLAA. On the 'other side' of the argument, issues that may slow down potential delivery include: 
 

- the need for S106 agreements or further submission of details 
- dealing with contaminated land/significant infrastructure requirements - dealing with environmentally sensitive locations - sites in multiple ownership - sites with significant opposition 
 
Again, this is not an exhaustive list. It is important not to be over-optimistic about site availability and delivery rates.  Evidence on delivery rates must be credible. Hold discussions with developers and key stakeholders. Sites may be able to be delivered when the market improves.  Do not include unrealistic rates in the current climate, but do not assume there will be no market recovery through the plan period. 

Q: Should the 5 year requirement include past surplus/deficit and if so how far back to? A: The 5 year requirement is about supply, not demand. Any past surplus/deficit starts from the beginning of the plan period. If the plan has yet to be adopted, then the period starts at submission. The Planning Practice Guidance covers the issue of surplus/deficit here: 
http://planningguidance.planningportal.gov.uk/blog/guidance/housing-and-economicland-availability-assessment/stage-5-final-evidence-base/ 
See more at: http://www.pas.gov.uk/pm-q-a-housing#sthash.RuSoVaef.dpuf Q: What is the role of the Strategic Housing Market Assessment in calculating need, and how do you account for backlog of under-delivery? 

 
A: Having an up-to-date, robust Strategic Housing Market Assessment should re-set the clock, and therefore carrying forward under-provision from a previous plan period would be `double counting'.  Make sure however that the Strategic Housing Market Assessment takes account of `backlog' which is unmet need for housing that still exists at the start of the new plan period (for example, the needs of the homeless and other households living in unacceptable accommodation).  The Strategic Housing Market Assessment should show all those in need.  It is therefore vitally important to have a properly done Strategic Housing Market Assessment that has the right scope. 'Backlog' should therefore be a short-term issue which can be dealt with through updating the Strategic Housing Market Assessment.  Once the plan period has started, any under-delivery against current targets is a 'shortfall' and needs to be addressed preferably within the first 5 years, as set out in the Practice Guidance. Figures based on census information that comes out in 2014 will be more helpful, as they will cover a longer period and are based on the full census data.  In the meantime, you need to work with what is currently available.  The interim household projections are useful as a comparison with the 2008 projections, but they only run to 2021. 

 
Q: Where you are using the Regional Strategy figure, how do you assess the amount of backlog, now that the Regional Strategy has been abolished? 

 
A:  Carry out an up to date Strategic Housing Market Assessment.  This should show all unmet need as well as newly-forming need. The importance of having this up-to-date information is illustrated in the judgement in respect of Zurich Assurance Limited v Winchester City Council [2014] EWHC 758 (Admin) (RT/38). The judgement states: '…There was no reason whatever for a person in 2011 seeking to draw up a current estimate of population growth and housing requirements looking into the future from that date to 2031 and using up-todate evidence to do so, to add on to the estimated figures any shortfall against what had been estimated to be needed in the first phase of the previously modelled period included in the South East Plan' (paragraph 92). The objectively assessed figure has to be the starting point. There are several useful resources: 
 

- The front end to the demographic projections for your area is the 'What 
Homes Where' toolkit: http://www.howmanyhomes.org/5.html  
 
- There is also a paper on assumptions, as part of this resource, written by 
Cambridge Centre for Housing and Planning Research: 
http://www.howmanyhomes.org/resources/Choice_of_Assumptions.pdf  
 When factoring in economic projections, a job-led plan must plan for the accompanying housing.  If you want more jobs in your area than the projections show, you have to look to accommodate the additional housing required to meet that job growth.  If not, you are essentially planning a commuter scenario, and this would have to be agreed with neighbouring and affected authorities.  You should also ask yourself 'would such an approach be sustainable?'. 

Q: Are 'broad locations' defined anywhere, and is it still appropriate to make an allowance for such sites in the 11-15 year period? 

A: The National Planning Practice Guidance clearly states that the following are acceptable: 

- Vacant and derelict land and buildings including empty homes, redundant 
and disused agricultural buildings, potential permitted development changes, for example offices to residential. 
- Additional opportunities in established uses, for example, making productive 
use of under-utilised facilities such as garage blocks. 
The Planning Practice Guidance covers this in this section: 
http://planningguidance.planningportal.gov.uk/blog/guidance/housing-and-economicland-availability-assessment/methodologystage-1-identification-of-sites-and-broadlocations-determine-assessment-area-and-site-size/ If we can assume this refers as much to broad locations as sites, then it effectively repeats what is in paragraph 46 of the DCLG SHLAA Practice Guidance of 2007 
where it refers to land 'within and adjoining settlements'. So: SHLAA sites + Broad Locations (plus any permissions and allocations not identified in the SHLAA) = total supply. 

Windfall sites are something that will be generally 'phased out', as, once you have a really robust SHLAA, you won't need to rely on windfalls anymore. That said, there will always be unidentified sites.  The key is whether you feel you can rely on the evidence to demonstrate they will continue to contribute to the 5-year supply. The Planning Practice Guidance covers the issue of windfall sites here: http://planningguidance.planningportal.gov.uk/blog/guidance/housing-and-economicland-availability-assessment/stage-3-determining-the-housing-potential-of-windfallsites-where-justified/ This supports the continued use of broad locations, and does not seem to discount the potential for built-up areas to contribute to this source in later years.  So you could identify 'Town Centre X' as a broad location, define on a map, and show expected contribution in years 6 - 15. 

 
Q: The Planning Policy Guidance suggests that any individual authority in the position of carrying out a Strategic Housing Market Assessment when others in the housing market area have recently adopted should build upon the existing work.  What does 'build upon' mean? 

 
A: You cannot ignore the new evidence.  Any adopted figures can't be set in stone and may need to be revised in the light of that new evidence. 

Q: How do you define 'persistent under-delivery' of housing? A: The Planning Practice Guidance deals with this issue in Paragraph 035 of this section: http://planningguidance.planningportal.gov.uk/blog/guidance/housing-andeconomic-land-availability-assessment/stage-5-final-evidence-base/ 
Q: When undertaking a new Strategic Housing Market Assessment there are obviously a number of scenarios that can be used.  How do you go about picking a number? A:  Inspectors will want to see a number.  All your work will come up with a range. Your 'chosen' number will fall somewhere within the range.  Inspectors will want to see the 'working out', or reasoning behind the eventual number chosen.  It should ideally sit behind all the reasonable assumptions you have made, so as to be a 'corroborated' figure.  In other words, it must align best with the assumptions. The obvious point is, if you are going for a certain level of additional job creation, you have to ensure you would be providing enough houses to meet that, even if it would mean providing more than the straight demographic trend data. Inspectors (and others) will mainly look for consistency in the figure chosen. You will also have to be satisfied that the final figure meets the requirements of the NPPF, in so far as you can demonstrate it will boost significantly the supply of housing. 

Q: Is there any detailed guidance on the methodology for producing housing trajectories? A: The Planning Practice Guidance does mention trajectories in this section: http://planningguidance.planningportal.gov.uk/blog/guidance/housing-and-economicland-availability-assessment/stage-4-assessment-review/ This is not detailed guidance.  Key points to consider are: 

- Have regular dialogue with developers regarding delivery rates - For individual sites, if you are relying on them to come forward sooner, you 
will need more evidence to support that assumption. This will include 
evidence regarding constraints and risks to delivery, such as contaminated land, or S106 negotiations. 

- Some authorities have considered a phased trajectory to deal with particular 
issues they are facing: 
o Severe, time-limited constraints 
o Reliance on large sites that will take time to deliver 
 
Q: Is the 15 years the core strategy period and therefore fixed or rolling as per the 5 year? A: The 15 year period for the trajectory should be fixed to the end date of the plan. If you revise the plan and therefore change the 'end date', you will need to adjust the trajectory accordingly. 

Q: What is the starting point for the 5 year - the current year or future years? A: The starting point should be the current year. Given that you have to demonstrate you have a 5 year supply at examination, this should be the 5 year period from the examination. If any sites end up going to appeal, where a lack of 5 year supply is being used as an argument by the applicant, then you are likely to have to present up to date information on the current state of supply. The Practice Guidance (paragraph 33) states 'local planning authorities should consider both the delivery of sites against the forecast trajectory and also the deliverability of all the sites in the five year supply.  By taking a thorough approach on an annual basis, local planning authorities will be in a strong position to demonstrate a robust five year supply of sites'.