Drop down rails to facilitate transfer to a WC are likely to be required to either side of the WC, whilst handrails and supports subject to a lower loading are likely to be required elsewhere. On that basis, it would be reasonable to expect the wall behind the WC to support a load of 1.5kn, but it may be reasonable for other walls to support a reduced load of 0.85kn. It would also be reasonable to design to meet these loads only in the zone where hand rails are likely to be fixed e.g. between 300-1500mm above finished floor level. Where boxing is not a reasonable fixing point, it may not need to be strengthened. Detail on location of wall strengthening can be discussed on a case by case basis with the Building Control Body.