Space standards and accessible housing as appraisal inputs

Space and accessibility standards hit an appraisal in two different places, and developers routinely argue the wrong one. The cost of building an M4(2) dwelling is modest and well evidenced. The cost of the nationally described space standard is mostly not a cost at all: it is a reduction in unit count on a fixed footprint, and it shows up in gross development value rather than in the cost plan.

Get that the right way round before the independent reviewer does. An appraisal that claims a large build cost uplift for space standards, without showing the unit count effect, invites a line-by-line challenge it will lose.

How an optional standard becomes a cost line

Neither standard applies automatically. The space standard operates only where a local plan policy adopts it, and Approved Document M4(2) and M4(3) apply only where a planning condition requires them. Planning Practice Guidance expects the policy to be justified by evidence of need, viability and timing before it can be imposed.

That justification is the first thing to check. Where a policy was adopted on an evidence base that predates a change in local values or costs, there is an argument about weight. Where the policy is recent and was itself viability-tested at plan stage, the argument is much weaker, because the council will say the testing has already been done.

Once the requirement bites, it enters the appraisal through the cost plan for accessibility and through the schedule of accommodation for space.

What the space standard actually requires

Built-in storage is counted within those areas, at 1 m² for a one person dwelling and 1.5 m² for two, rising by half a square metre per additional person. A double bedroom must be at least 11.5 m² and 2.75 metres wide, a single 7.5 m² and 2.15 metres wide. At least 75 per cent of the gross internal area must have a floor to ceiling height of 2.3 metres or more.

Multi-storey dwellings have their own figures, and the standard should be read in full rather than from a table like this one.

Space standards and the unit count

Here is the arithmetic that matters. On a site where the massing is fixed by daylight, height and townscape constraints, the total sellable area is roughly fixed. Applying the space standard to a scheme designed below it does not add area, it redistributes it, and the scheme loses units.

Losing units reduces gross development value more than it reduces cost, because a proportion of the cost is area-driven and only a proportion is unit-driven. Kitchens, bathrooms, entrance doors and service connections are unit-driven, so fewer larger units cost slightly less to build per square metre while generating less revenue per square metre in most markets.

That last point is the one to evidence. Value per square metre usually falls as unit size rises within a submarket, so a 70 m² two bedroom flat rarely sells for the same rate as a 61 m² one. Produce the comparables by size band rather than a single blended rate, and the effect becomes visible.

Where the massing is not fixed, the argument is much weaker. A scheme that could grow to absorb the standard, and chooses not to, is making a design decision rather than facing a viability constraint.

Accessible and adaptable dwellings

M4(2) covers approach routes, level or gently sloping access, wider doorsets and corridors, a WC at entrance level, bathroom walls capable of taking grab rails, and provision for a future through-floor lift in houses over one storey.

The cost is real and it is modest per dwelling, and it is best evidenced from a published source. The government's impact assessments accompanying the Part M consultations set out per-dwelling estimates, and current build cost data services give rates by dwelling type. Use one of those rather than an in-house figure, because a reviewer will not accept a number with no provenance.

Two design points carry more cost than the specification suggests. The lift provision in houses affects the plan rather than just the budget, because the stacked space has to exist. And level threshold detailing on a sloping site can drive external works and retaining structures that dwarf the internal cost.

Government has stated an intention to make M4(2) the baseline standard in England, which would require amending Part M. Verify the current position on gov.uk before fixing the cost plan, because once that happens the policy argument disappears and the cost sits in the base build rate for everyone.

Wheelchair user dwellings

M4(3) is a different order of cost. It requires larger areas, turning circles, a wheelchair accessible bathroom, hoist provision in the adaptable version, wider circulation and an accessible parking space, and in flats it requires a lift and a compliant approach from the parking to the entrance.

Boroughs applying the London Plan require 10 per cent of dwellings to meet M4(3) with the remainder at M4(2), and other authorities set their own proportions. The appraisal treatment should separate the per-dwelling construction uplift from the area uplift, because the area uplift interacts with the unit count in the same way the space standard does.

Where the requirement is expressed as a percentage, the distribution across the unit mix matters. Putting all the wheelchair user dwellings in the largest unit type is expensive. Spreading them sensibly across types and cores, with the accessible parking close to a core, usually costs less.

Where councils push back, and why they often win

The standard response is that policy requirements should already be reflected in the land value. Planning Practice Guidance takes that position, and it is the reason the benchmark land value dominates an independent review. If the space standard and the accessibility policy were in the adopted plan when the land was acquired, a reviewer will say the price paid should have accounted for them, and that the shortfall is a land price problem rather than a policy problem.

The counter-argument that works is scheme-specific: a site where the massing is constrained by daylight or townscape, where the comparable evidence shows value per square metre falling with unit size, and where the cost plan itemises the uplift from a published source. The argument that does not work is a blended percentage added to build cost with no breakdown.

Illustrative example. A 42 unit scheme redesigned to meet the space standard within the same envelope becomes 38 units. The cost plan falls slightly, the gross development value falls by more, and the deficit is around the value of two units. Presented as a unit count effect with a test-fit drawing and sales evidence by size band, the point is accepted. Presented as a cost uplift per square metre, it is not. The figures are illustrative.

Testing it properly in the sensitivity analysis

Run four cases, not two: policy-compliant with standards, policy-compliant without, reduced affordable provision with standards, and reduced provision without. That isolates which requirement is doing the work, which is the question the officer actually has to answer.

Then supply the test-fit drawings alongside the appraisal. A schedule of accommodation showing both layouts is worth more than a page of narrative, and it also feeds directly into the Design and Access Statement and the Affordable Housing Statement. Where the matter is likely to run to an inquiry, the disclosure position needs thinking about early, and viability at appeal sets out how little of an appraisal stays private.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.

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