The tenth dwelling: crossing into major development

Going from nine to ten dwellings changes the application you are making, not just the number on the form. Ten dwellings is the definition of major development in Article 2 of the Development Management Procedure Order 2015, and crossing it moves the determination period from eight weeks to thirteen, brings in newspaper publicity, makes a Design and Access Statement compulsory, engages affordable housing policy in most areas, and switches the biodiversity calculation from the small sites route to the full statutory metric.

For a self-builder or small developer deciding whether to squeeze a tenth plot onto a site, that is a real commercial question. The extra unit has to pay for several months of programme and a thicker evidence base, and on a tight site it often does not.

What changes the moment the tenth unit appears

Major development for residential purposes means ten or more dwellings, or a site of 0.5 hectares or more where the number of dwellings is not yet known. The second limb catches outline applications on half-hectare sites even where the eventual number is lower.

Not everything changes at ten. The community infrastructure levy is charged on floorspace rather than unit count, so it applies either side of the line. A Statement of Community Involvement is a feature of much larger schemes. And environmental impact assessment screening for urban development projects does not bite until well above this threshold, at more than 150 dwellings or a site area over 5 hectares under Schedule 2 to the 2017 Regulations.

Publicity, consultation and the determination period

The thirteen week period is the headline change and the easiest to underestimate. In practice the real elapsed time is longer, because statutory consultees have their own response periods and the extended publicity adds a fortnight at the front end.

Three consultation consequences matter on a small major scheme:

• The lead local flood authority becomes a statutory consultee on surface water drainage, which means a drainage strategy that would have satisfied a case officer now goes to a drainage engineer who will read the calculations.
• Newspaper publicity reaches a wider audience than neighbour letters, so expect more representations and allow for a committee report that has to address them.
• Highway authority engagement tends to deepen, even where the trip generation is modest, because the application is categorised differently in their system.

Plan the programme backwards from the thirteen weeks and add time for a committee cycle. Whether a scheme of ten houses goes to committee depends on the authority's scheme of delegation, and the shift described in scale and the move from delegated to committee decisions is one of the least predictable parts of the process.

Affordable housing policy engaging

National policy says affordable housing contributions should not be sought from developments of fewer than ten homes, and of no more than 1,000 square metres of gross floorspace. In designated rural areas authorities may set a lower threshold, sometimes as low as five units or fewer, so check the adopted policy for your area rather than assuming the national position.

Cross the threshold and the local plan requirement applies. That may be on-site provision, a commuted sum, or a mix, and the percentage varies widely between authorities.

Two practical points. The requirement is usually calculated on the total, not on the units above the threshold, so a scheme of ten units can attract a contribution on all ten. And the arithmetic rarely produces a whole number of affordable units at this scale, which is why small majors so often end up with a financial contribution instead.

An Affordable Housing Statement setting out the tenure, the mix and the mechanism is normally expected with the application. Where the contribution makes the scheme unviable, that is a viability case with its own evidence requirements, and it is a slow route on a scheme of this size.

Biodiversity gain under the majors regime

Mandatory biodiversity net gain applies to both minor and major development, but the route differs. Small sites can use the small sites metric, a simplified version intended to reduce cost. Major development uses the full statutory metric.

The practical differences are the survey detail required, the habitat condition assessment, and the cost of the assessor's time. A full metric assessment needs a habitat survey mapped to the required classification and condition assessed against published criteria, which is a day in the field and several days at a desk rather than a quick appraisal.

The gain plan also has to be approved by the local planning authority before development begins, and the thirty year management and monitoring obligation is secured by a conservation covenant or a planning obligation. A Biodiversity Net Gain Assessment must be completed by a competent person under the Environment Act 2021 regime, and PlanScope's draft is a starting point for that professional rather than a substitute for them.

Budget for the metric work at the point you decide on the tenth unit, not afterwards. Finding out that the site scores badly on habitat condition after the layout is fixed removes the cheapest options.

Fees and the cost step

Planning fees for major residential development run on a banded scale, with a base amount covering the first tranche of dwellings and an additional amount per dwelling above it. The figures are revised and indexed, so take them from the current published schedule rather than from a previous project.

The fee itself is rarely the decisive number. The real cost step is the evidence base:

1. A Design and Access Statement, which becomes compulsory.
2. A full metric biodiversity assessment rather than a small sites assessment.
3. A drainage strategy written for scrutiny by a drainage engineer rather than a planning officer.
4. An affordable housing position, and possibly a viability appraisal to argue against it.
5. Transport evidence appropriate to the new category, even if a full assessment is not triggered.
6. Utilities capacity, where a tenth unit can tip a connection from a simple service to a reinforcement. The point made in utilities capacity checks before the unit number is fixed applies precisely here.

A Design and Access Statement is not a box-ticking document on a major scheme. It carries the design rationale that the committee report will quote, so it is worth writing properly rather than treating as an attachment.

Adding a plot to an existing permission

A common sequence: nine dwellings permitted, then a desire to add a tenth. The route is not a variation.

Where the description of development states a number of dwellings, that number cannot be increased by a section 73 variation, because the Court of Appeal in Finney v Welsh Ministers confirmed that a condition cannot be varied so as to conflict with the description. Nor can a non-material amendment under section 96A change the description.

So the options are a fresh full application for the whole site at ten units, or a separate application for the additional plot. The second looks attractive and carries a risk: the authority may treat the two together as a single scheme of ten, and apply the major development requirements retrospectively to the combined development. Authorities are alert to deliberate subdivision, and the issue has been litigated in various forms.

If the tenth unit is part of the same scheme in substance, apply for ten and accept the consequences. A piecemeal approach that collapses at validation costs more than the honest route.

Deciding whether the extra unit pays

Run the numbers before the design. Four figures decide it:

The gross development value of the additional unit, net of the build cost and the finance cost over a longer programme. The affordable housing contribution attributable to the whole scheme, not just the extra unit. The additional professional fees for the thicker evidence base. And the cost of five extra weeks of determination plus any committee delay, expressed as finance on the land.

On a scheme of ten modest houses, the affordable housing contribution alone can exceed the margin on the tenth unit. That is the calculation that most often sends schemes back to nine.

The honest answer varies by authority and by site. Where the local affordable housing policy is a modest commuted sum and the site has good habitat condition scores, ten can work comfortably. Where the policy seeks a percentage of a rising value and the site is species-rich grassland, nine is usually the better scheme.

Enter your postcode on PlanScope to see which reports a scheme is likely to need on each side of the ten dwelling threshold.

This is general guidance, not legal advice; check the adopted policy and the current fee schedule 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.