Counting units where an existing dwelling is demolished
Demolish and rebuild, and gross and net unit counts stop agreeing. Here is which figure the major development test, affordable housing policy, the Community Infrastructure Levy and the biodiversity gain condition each rely on.
Demolish one house and build eleven, and you have a scheme that is major for the purposes of validation and consultation, but which produces a net gain of ten dwellings for affordable housing purposes and may attract levy liability on rather less floorspace than you built. Net units on a demolition and replacement site are not one number. They are three or four different numbers, each defined by a different instrument, and the arithmetic you put on the application form is only one of them.
The rule to hold on to is this: the major development test counts homes provided, the affordable housing threshold in most adopted local plans counts net additional homes, the Community Infrastructure Levy counts net chargeable floorspace with a deduction for qualifying existing buildings, and biodiversity net gain ignores dwellings entirely and counts habitat.
Gross and net counts and which applies where
A gross count is the number of dwellings that will exist on completion. A net count is that figure minus the dwellings lawfully in existence beforehand. On a site with one bungalow replaced by six flats, the gross figure is six and the net figure is five.
Most validation and procedural triggers run off the gross figure, because the question they ask is how much development is being built, not how much is being added to the housing stock. Most planning obligation triggers run off the net figure, because the question there is what the scheme contributes. Confusing the two is the commonest arithmetic error on small demolition sites, and it usually surfaces as a validation query rather than a refusal.
The application form asks for both. Complete the existing and proposed dwelling numbers honestly and the council can derive whichever figure it needs.
Major development and the unit definition
The glossary to the National Planning Policy Framework, in the edition published on 17 August 2026, defines major development for housing as development where 10 or more homes will be provided, or where the site area is 0.5 hectares or more and the number of homes is not known. Article 2 of the Town and Country Planning (Development Management Procedure) (England) Order 2015 carries the procedural definition used for determination periods and publicity.
"Provided" is a gross word. A scheme that demolishes two houses and builds eleven provides eleven homes and is major, even though it adds nine. That matters well beyond a label: major status brings the 13-week determination period, wider publicity, a design and access statement requirement for certain applications, and for the authority a different set of consultation duties. It is also the trigger the Statement of Community Involvement hangs off in many local validation lists.
The 0.5 hectare limb only operates where the number of homes is unknown, which in practice means outline applications with no indicative capacity. Do not assume a 0.6 hectare site with a stated nine dwellings is major on area alone.
Affordable housing thresholds and net gain
National policy restricts affordable housing contributions to major development, with exceptions for designated rural areas where authorities may set a lower threshold. Beyond that, the figure and the basis come from the adopted local plan, and most plans express the trigger as net additional dwellings.
That wording does real work on demolition sites. A scheme replacing four flats with thirteen provides thirteen and nets nine, so it is major but may sit below a ten-unit net affordable housing threshold. The opposite also happens where a plan is drafted in gross terms. Read the policy, not the summary table in the local plan's appendix, and where the drafting is ambiguous put your reading in writing in the Affordable Housing Statement rather than leaving the officer to pick.
Two further points catch people. Vacant building credit can reduce the affordable requirement where a vacant building is brought back into use or demolished, and its application is contested and heavily dependent on the local plan. And some authorities apply their threshold to the whole of a site that has been artificially subdivided, so phasing a twelve-unit site into two sixes rarely works.
Levy calculation and demolished floorspace
The Community Infrastructure Levy is charged on net chargeable floorspace under the Community Infrastructure Levy Regulations 2010 as amended, using gross internal area rather than unit counts. Existing floorspace is deducted where the building qualifies as an in-use building, which turns on lawful use for a continuous period of at least six months within the three years ending on the day planning permission first permits the development.
Get that test wrong and the bill moves by a large amount. A long-empty commercial building demolished for flats will often fail the in-use test, so none of its floorspace is deductible, while a house occupied until shortly before determination will usually pass. The evidence is documentary: council tax or business rates records, utility accounts, tenancy agreements.
| Instrument | What it counts | Effect of demolition |
|---|---|---|
| Major development test | Homes provided (gross) | No deduction for demolished dwellings |
| Affordable housing policy | Usually net additional homes | Demolished dwellings normally deducted |
| Community Infrastructure Levy | Net chargeable floorspace | Deducted only if the in-use test is met |
| Biodiversity gain condition | Habitat units on site | Baseline includes gardens and hardstanding |
| Application fee | Number of dwellings created | Check the current fee schedule |
Self-build and residential annexe or extension exemptions from the levy exist and are claimed on prescribed forms before work starts. Miss the sequence and the relief is lost, which is an administrative failure rather than a planning one.
Biodiversity gain and the pre-development baseline
The biodiversity gain condition under Schedule 7A to the Town and Country Planning Act 1990, inserted by the Environment Act 2021, takes no interest in how many dwellings you demolish. It measures habitat on the site before development using the statutory metric and requires a 10 per cent gain, secured for 30 years through an approved biodiversity gain plan.
Demolition sites behave counter-intuitively here. Sealed surfaces and the building footprint score nothing, so knocking a house down costs you very little baseline. The garden is the problem. A large mature garden can carry a meaningful number of units as modified grassland, introduced shrub and individual trees, and replacing that with flats, parking and a communal amenity strip frequently produces a shortfall that has to be met off site or through statutory credits.
The exemptions matter on small demolition sites: householder applications, self-build and custom housebuilding of nine dwellings or fewer on a site under 0.5 hectares, and the de minimis threshold for developments affecting less than 25 m² of on-site habitat or 5 metres of hedgerow. A licensed or competent person is expected to complete the statutory metric, and a Biodiversity Net Gain Assessment drafted in advance is a starting point for that person rather than a substitute.
Where the counts diverge on one site
Take a site with two semi-detached houses, both occupied, totalling around 180 m² of gross internal area, replaced by twelve flats totalling 850 m², on 0.28 hectares with large rear gardens. Illustrative figures, but the shape is common.
The scheme provides twelve homes, so it is major. It nets ten, so a net-based affordable housing threshold of ten is engaged. Levy liability is charged on roughly 670 m² if both houses pass the in-use test, and on 850 m² if they do not. Biodiversity baseline is driven almost entirely by the gardens, not the buildings. And the fee is calculated on the number of dwellings created under the current fee regulations, which are indexed annually, so check the schedule in force on the day you submit.
Four instruments, four answers, one site.
Stating the arithmetic in the application
Set the numbers out once, in a table, in the covering document, and then keep every other document consistent with it. Existing dwellings, proposed dwellings, net change, existing gross internal area with the basis of measurement, proposed gross internal area, site area to the red line, and the in-use position on each building to be demolished.
Where you are relying on a reading of a local plan threshold, say so and give the reasoning in the Planning Statement. Officers challenge silent assumptions far more often than stated ones. More detail on what else changes as a scheme grows is in our note on housing mix and space standards as schemes grow.
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.
Frequently asked questions
Is a scheme that demolishes two houses and builds eleven major development?
Yes. The National Planning Policy Framework glossary counts homes provided, not homes added, so eleven homes provided makes the application major even though the net gain is nine. That brings the 13-week determination period and the wider publicity and consultation requirements that go with major status.
Do I get credit for demolished floorspace when the levy is calculated?
Only if the building qualifies as an in-use building under the Community Infrastructure Levy Regulations 2010. The test is lawful use for a continuous period of at least six months within the three years ending on the day planning permission first permits development. Keep council tax, rates or tenancy evidence to prove occupation.
Does demolishing a house reduce my biodiversity baseline?
Barely. The building footprint and hardstanding score close to nothing in the statutory metric. The baseline on most demolition sites comes from the garden, so mature grassland, shrub and trees are what you have to replace. Large rear gardens are the usual reason a small replacement scheme cannot reach 10 per cent on site.
Which figure does the planning application fee use?
The fee for new dwellings is based on the number of dwellings created, under the fee regulations in force on the day of submission. Those figures are indexed annually, so check the current schedule rather than reusing a figure from an earlier application. The council will hold the application invalid if the fee is short.
Can I split a twelve-unit site into two six-unit applications to stay below a threshold?
Rarely with success. Authorities routinely treat artificially subdivided sites as one development for threshold purposes, and appeal decisions have supported that approach where the parcels share an access, a promoter or a single planning history. Expect the affordable housing and biodiversity positions to be assessed on the whole site.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.