Housing mix: how councils set unit size policy

Housing mix planning policy tells you how many one, two, three and four bedroom homes a council expects on a site of your size, and it is one of the few policy tests where a scheme can be technically compliant on every other measure and still be recommended for refusal. The requirement sits in the adopted local plan, usually as a table or a set of percentages, and it is drawn from the authority's housing needs evidence rather than from national policy.

The starting point for any developer is therefore the local plan, not the NPPF. National policy asks plan makers to assess and reflect the size, type and tenure of housing needed for different groups. It does not set the split. Your council does.

Where mix requirements come from

Most local plans carry a policy along the lines that residential development should provide a mix of dwelling sizes reflecting the needs identified in the authority's most recent housing needs assessment. Some plans put the percentages in the policy itself. Others put them in a supporting table, or in a housing supplementary planning document, which changes how much weight they carry.

That distinction is worth understanding. A percentage written into an adopted development plan policy engages section 38(6) of the Planning and Compulsory Purchase Act 2004 and the application must be determined in accordance with it unless material considerations indicate otherwise. A percentage in a supplementary planning document is a material consideration and no more, so the argument for departing from it is easier.

Check the age of the evidence as well. A mix table drawn from an assessment published seven years ago carries less weight than one refreshed last year, particularly where the housing market has moved since.

How needs evidence sets the bedroom split

The bedroom split comes from a strategic housing market assessment or a local housing needs assessment, which models demographic change, household formation, existing stock, overcrowding and under-occupation, then projects the size of homes required across the plan period.

Those models produce a split for market housing and a separate split for affordable housing, and the two usually differ sharply. Affordable need typically skews towards two and three bedroom family homes. Market need in an urban authority often skews towards one and two bedroom flats. A policy that applies a single split to both tenures is usually a sign that the evidence has been simplified.

Read the assessment itself rather than the summary table in the plan. It will tell you which submarket your site sits in, and whether the authority modelled anything specific about flats, older people's housing or houses in multiple occupation. That detail is the raw material for any departure argument you later want to run, and it belongs in the Planning Statement rather than in a covering letter.

Space standards and internal areas

Separately from the mix, most authorities apply the nationally described space standard, published by government in 2015 and setting minimum gross internal areas by the number of bedrooms and bedspaces. A one bedroom, one person flat is 39 m2, a one bedroom two person flat is 50 m2, a two bedroom three person dwelling is 61 m2 and a two bedroom four person dwelling is 70 m2, with figures rising by storey count for houses.

The standard also fixes bedroom dimensions: a single bedroom of at least 7.5 m2 and 2.15 m wide, a double of at least 11.5 m2 and 2.75 m wide, and a floor to ceiling height of at least 2.3 m over three quarters of the gross internal area.

Two points are frequently misunderstood. The standard is optional, and an authority can only require it through a local plan policy justified by evidence of need, viability and timing. And the standard is expressed as a minimum, so a scheme that meets it exactly on every unit is compliant, if unimpressive.

Since 2021 the space standard has also applied to new dwellings created under permitted development rights, which closed the gap that produced very small conversions. If your scheme is going down the prior approval route, check that the internal areas work before you rely on any of the other permitted development rights.

Accessible and adaptable dwelling percentages

Part M of the Building Regulations sets three categories: M4(1) as the baseline, M4(2) accessible and adaptable dwellings, and M4(3) wheelchair user dwellings. As with space standards, higher categories apply only where a local plan policy requires them and evidence justifies the requirement.

In London the London Plan requires 90 per cent of new homes to meet M4(2) and 10 per cent to meet M4(3), with wheelchair accessible units in the affordable tenure to be wheelchair accessible rather than adaptable. Outside London the percentages vary widely, and some authorities have no policy at all.

These requirements bite on layout. M4(2) affects entrance thresholds, corridor widths, WC provision and the ability to fit a future stairlift. M4(3) affects turning circles, hoist routes and lift sizes. Retrofitting them after the mix is fixed is expensive, so test the standard against the unit types at the earliest design stage.

Arguing a departure from the policy mix

Councils do accept departures. What they do not accept is a departure justified only by what the developer would prefer to build.

Arguments that carry weight, in rough order of strength:

1. Site characteristics that make the policy mix physically undeliverable, such as a constrained urban plot where family homes with private gardens cannot be provided at any reasonable density.
2. A viability case, prepared in accordance with the RICS professional standard and tested against the PPG on viability, showing that the policy mix would render the scheme unviable. This runs alongside the Viability Assessment and the affordable housing position.
3. More recent or more granular local evidence than the plan relies on, for example a housing needs assessment published since adoption that shows a different split for this submarket.
4. The characteristics of the intended occupiers, where the scheme is specialist in fact, such as older people's accommodation or supported housing.
5. An overall package argument, where a departure on mix is offset by over-delivery on affordable housing, tenure or accessible units.

Weak arguments include comparisons with what neighbouring authorities require, assertions about what the market wants without evidence, and the observation that a previous application on the site was approved with a different mix.

What to put in the planning statement

Set out the policy, the evidence behind it, your scheme's mix and the comparison, in that order, with a table. Then deal with the gap head on.

State the percentage sought by policy for each unit size, the percentage proposed, and the difference expressed in units rather than only in percentages, because on a small scheme a 10 per cent shortfall is often a single flat. Explain the site constraint or evidence that drives the difference. Where the mix departs on market housing but complies on affordable, say so clearly, and carry the affordable position through into the Affordable Housing Statement so the two documents agree.

Add the space standard schedule as an appendix: unit reference, bedrooms, bedspaces, proposed gross internal area, required area, and the difference. Officers check this line by line, and a schedule they can verify in five minutes removes an entire round of correspondence.

Browse the report types your scheme is likely to need on PlanScope and draft the housing mix narrative against the adopted policy table.

This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.