Curtilage coverage for a garden outbuilding: worked example

The 50 per cent curtilage coverage test is the limit most garden room projects fail on, and almost nobody measures it before ordering the building. Under Class E in Part 1 of Schedule 2 to the General Permitted Development Order 2015, an outbuilding is not permitted development if the total area of ground covered by buildings, enclosures, pools and containers within the curtilage, other than the original dwellinghouse, would then exceed half the area of the curtilage excluding the ground area of the original dwellinghouse.

Read that twice. The original dwellinghouse comes out of both sides of the sum, and everything added since goes into the top. A 1990s rear extension counts against you even though it is part of the house you live in.

What the coverage test actually measures

The test is a ratio of ground areas, measured in plan. It asks what proportion of your garden is already built on, treating the original house as neither numerator nor denominator.

Expressed plainly: take the curtilage area, subtract the ground area of the original dwellinghouse, and that is the figure you are allowed to cover half of. Then add up the ground area of every building, enclosure, pool and container in the curtilage apart from the original dwellinghouse, including any extensions to it, and compare.

The same test appears in Class A for house extensions, with the same wording. So an extension and an outbuilding compete for the same allowance, and the order you build them in does not change the arithmetic.

Establishing the curtilage boundary

The Order does not define curtilage. The courts have treated it as the land that forms part and parcel with the dwelling and serves its purposes, which for a typical suburban plot means the whole of the front and rear garden inside the title boundary.

Where it gets interesting is larger plots. A paddock beyond a post-and-rail fence, a strip of woodland separated by a hedge and gate, or land that was once agricultural and has never been brought into domestic use may fall outside the curtilage even though you own it. Including it inflates your allowance and is the quickest way to be caught out later.

Three indicators point the same way on most sites: enclosure with the house as a single parcel, domestic use in fact, and function serving the dwelling. If you are relying on land that fails two of those tests, assume the council will too.

Measure from the title plan, then check it against what is on the ground. Fences move.

Excluding the original dwellinghouse footprint

The original dwellinghouse means the dwelling as it was built, or as it stood on 1 July 1948 for anything older. Not as it stands today.

For a 1935 semi, that means working out the footprint before the 1970s kitchen extension and the 2008 rear addition. Historical mapping, old deeds plans, previous planning permissions for the extensions and the building control records between them usually settle it. Where the evidence is thin, a surveyor can read the structure and date the additions from the brickwork and the foundations.

This is the step people skip. Treating today's footprint as the original one both shrinks the denominator and removes the extensions from the numerator, which produces an answer that is wrong twice over in your favour.

What counts as a building for the test

The wording covers buildings, enclosures, pools and containers. In practice that brings in:

• extensions to the house built after the original dwellinghouse
• detached and attached garages, carports with a roof, and car ports
• sheds, garden rooms, home offices, summerhouses and greenhouses
• swimming pools, hot tub enclosures and pond structures with sides
• oil tanks, LPG tanks and large stores
• walled or fenced enclosures where they form a structure rather than a boundary

Boundary walls and fences along the plot edge are not usually counted as enclosures for this purpose, because they define the curtilage rather than cover ground within it. An internal walled compound does count.

Decking is the common argument. Raised decking with enclosed sides behaves like a structure and is usually counted. A flush timber platform laid on the ground behaves like a hard surface and is usually not.

Hard surfaces and why they are excluded

Patios, paths, driveways and parking areas do not cover ground with a building, enclosure, pool or container, so they fall outside the 50 per cent test entirely. You can pave the whole garden and still have your full outbuilding allowance.

They are controlled separately. Class F permits hard surfaces within the curtilage, and a front garden hard surface over 5 m² must be porous or drain to a permeable area within the curtilage, which is a drainage requirement rather than a coverage one.

That split catches people who assume a big driveway has used up their allowance. It has not.

Running the calculation

Six steps, in this order:

1. Measure the curtilage area in plan from the title plan, excluding any land outside the curtilage.
2. Establish the original dwellinghouse footprint and its ground area.
3. Subtract step two from step one. This is the base area for the test.
4. Halve the base area. This is your permitted coverage.
5. Add up the ground areas of all existing buildings, enclosures, pools and containers in the curtilage, excluding the original dwellinghouse but including every extension to it.
6. Add the proposed outbuilding footprint and compare the total with step four.

Measure externally, to the outside face of walls, and include any overhanging eaves where they form part of the building footprint in plan.

An illustrative calculation with a garage and a garden room

The following worked example is illustrative only. A 1930s semi on a plot of 340 m² inside the title boundary, all of it in domestic use and enclosed with the house.

Sixty-three square metres against an allowance of 136 m². The coverage test is satisfied with room to spare, and the project turns on the other Class E limits instead: single storey, maximum eaves height 2.5 m, maximum overall height 4 m with a dual-pitched roof or 3 m otherwise, and 2.5 m overall where any part sits within 2 m of a boundary.

Now change one figure. The same house on a 150 m² plot, which is common on a terrace, gives a base area of 82 m² and an allowance of 41 m². The existing 43 m² already exceeds it, and no outbuilding at all is permitted development. That is the case where the household either reduces the proposal and applies for planning permission, or removes the shed.

Designations change the answer again. On land in a conservation area, National Park, National Landscape or the Broads, an outbuilding at the side of the house is not permitted, and any building more than 20 m from the house is limited to 10 m² of ground area. In the curtilage of a listed building, Class E does not help at all and listed building consent is needed.

Where a protected tree sits near the proposed building, the root protection area may govern the position regardless of the coverage sum, and a tree protection plan and method statement is what the council will want to see.

When the arithmetic says no

If the proposal exceeds the allowance, it needs planning permission as a householder application. The supporting documents are modest: drawings, a site plan and a short Planning Statement addressing the local plan's amenity and design policies. Talking to the neighbours first helps, and our neighbour engagement template gives a structure for that conversation.

If the arithmetic says yes and you want certainty, a lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that proposed works are lawful. A builder's opinion, a supplier's brochure and a planning officer's phone call are none of them binding.

Enter your postcode on PlanScope to see which reports your site needs. 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.