The 50 per cent curtilage test on a subdivided plot
The coverage test limits buildings other than the original dwellinghouse to half the curtilage. Subdividing a plot shrinks the denominator permanently.
The coverage test is the limit that quietly kills more permitted development schemes on small plots than height or depth ever does. Under Schedule 2 Part 1 to the GPDO 2015, the total area of ground covered by buildings within the curtilage of a dwellinghouse, excluding the original dwellinghouse itself, must not exceed 50 per cent of the area of the curtilage, again excluding the ground area of the original dwellinghouse. It appears as A.1(b) for extensions under Class A and as E.1(b) for outbuildings under Class E.
On a plot that has been subdivided, or where garden land has been sold off, the denominator shrinks while the buildings stay where they are. That is how a house with a 1990s extension and a double garage can find itself with no permitted development rights left at all.
How the coverage test works
The calculation has two terms, and getting either wrong produces a wrong answer.
The denominator is the area of the curtilage as it exists when the works are carried out, minus the ground area of the original dwellinghouse. The numerator is the total ground area covered by buildings, enclosures, pools and containers within that curtilage, again excluding the original dwellinghouse, and including everything already there plus whatever you propose to add.
So the sum is: existing buildings other than the original house, plus the proposed building, divided by curtilage area minus the footprint of the original house. If that exceeds 0.5, the works are not permitted development.
The phrase "original dwellinghouse" does the heavy lifting. For a house built after 1 July 1948 it means the house as originally built. For an older house it means the house as it stood on 1 July 1948. Every extension added since then counts in the numerator, not in the excluded footprint, which is why a much-extended house has a smaller allowance than its owners expect.
What counts as a building for the test
The wording is "buildings, enclosures, pools and containers", which is broader than people assume. Items that count:
- extensions to the dwellinghouse added after the original build
- detached garages, sheds, summerhouses, garden offices and greenhouses
- swimming pools and the ground area of hot tub enclosures
- storage containers and shipping containers
- enclosures such as walled or roofed structures, kennels and animal shelters
Items that generally do not count as ground covered: open hard surfacing and driveways, which fall under Class F, boundary fences and walls, which fall under Part 2, and open decking, though a raised platform brings its own separate restriction.
The treatment of a carport or an open-sided structure is less clear-cut and depends on whether it reads as a building in the ordinary sense. Where there is doubt, measure it both ways and see whether the answer changes. If it does, that is a question to put to the authority rather than to resolve optimistically.
The curtilage as it stands today
Curtilage is not defined in the GPDO and is a question of fact. It is broadly the land that serves the dwellinghouse as part and parcel of it: the garden, yard, parking and domestic outbuildings. It is not the same as the title boundary, and it is not the same as the planning unit.
Two consequences follow. A paddock or field in the same ownership, used for grazing rather than as garden, is generally outside the curtilage, so including it in the denominator to improve the arithmetic is wrong. And land that was once garden but has been fenced off, sold or changed in use is no longer part of the curtilage, so it drops out of the denominator even though the deeds may tell a different story.
The test applies to the curtilage as it exists at the time of the proposed works. That is the point that catches people after a subdivision.
Selling off garden land and the consequence
Work through an illustrative case. A detached 1970s house has an original footprint of 90m² on a plot whose curtilage measures 900m². Existing buildings beyond the original house are a 30m² rear extension and a 36m² detached garage, 66m² in total.
| Stage | Curtilage less original house | Buildings counted | Coverage |
|---|---|---|---|
| Before subdivision | 810m² | 66m² | 8 per cent |
| After selling 500m² for a new plot | 310m² | 66m² | 21 per cent |
| After a further 60m² outbuilding | 310m² | 126m² | 41 per cent |
| After a 40m² wraparound extension | 310m² | 166m² | 54 per cent |
The figures are illustrative, but the shape of the problem is real. Each parcel of garden sold reduces the denominator permanently, and the buildings already standing do not shrink to match. The last row is not permitted development, and the owner would need a planning application for the extension even though every other Class A limit is satisfied.
The sequence matters too. Had the outbuilding and extension been built before the land sale, they would have been lawful when carried out, and the sale would not make them unlawful. It simply removes the headroom for anything further.
Where a new plot has no curtilage history
A newly created plot raises a different question. Until a dwelling is built on it there is no dwellinghouse and so no curtilage of a dwellinghouse, and Part 1 rights do not exist. Once the house is built, that house is the original dwellinghouse and its curtilage is the land that serves it.
This is usually good news for the owner of the new plot: the coverage test starts from zero because there are no buildings other than the original dwellinghouse. But two traps are common. Permissions for new dwellings frequently remove or restrict Part 1 rights by condition, particularly on infill plots and in sensitive locations, and Article 3(4) means permitted development cannot override a condition. And where the new house was granted with a garage shown on the approved plans, that garage counts in the numerator from day one.
Read the decision notice for the new plot before assuming the allowance. Our note on garden offices and the separate planning unit problem covers the related question of when a garden building stops being incidental to the house.
Measuring and recording the calculation
Measure from a scaled survey, not from an aerial image. The usual sources of error are taking the curtilage from the title plan rather than from the land that actually serves the house, measuring building footprints to the inside face rather than to the external envelope, and forgetting a structure at the end of the garden.
Record the calculation in a short note with a plan attached. The plan should show the curtilage edged in red with its area stated, the original dwellinghouse hatched with its footprint stated, and every other building numbered with its footprint in a schedule. Keep the title deeds showing the date and extent of any land disposal with it.
That note is what you produce if an enforcement officer calls, and it is the core of a certificate application if you want formal confirmation. 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, and the coverage calculation is exactly the kind of question it is designed to settle. Where a certificate or an application is needed, a short planning statement setting out the calculation and the curtilage history does the job.
Why this is often the binding constraint
On a subdivided or compact plot the other Class A limits are rarely the problem. A 3m single-storey rear extension on a semi, 4m on a detached house, within the height limits and in matching materials, is unremarkable. The coverage test is different because it is cumulative across decades and across owners.
It bites hardest in three situations. A 1930s semi with a 1960s rear addition, a 1990s conservatory and a garden room has almost no headroom left. A plot where a new dwelling has been built in the garden leaves the original house on a fraction of its former land. And on a smallholding the house often sits on a small curtilage with substantial outbuildings that turn out to be inside it rather than outside.
Do the arithmetic before you pay for drawings. It takes twenty minutes with a survey and it decides whether you are in the permitted development world or the application world. Our explainers on the 50 per cent curtilage rule for garden outbuildings and on Class Q structural works cover the adjacent tests that often come up on the same site.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
What is the 50 per cent curtilage rule?
Under Schedule 2 Part 1 to the GPDO 2015, buildings, enclosures, pools and containers within the curtilage of a dwellinghouse, excluding the original dwellinghouse, must not cover more than half the curtilage area, which is itself measured excluding the original house footprint. It appears as A.1(b) for extensions and E.1(b) for outbuildings.
What counts as the original dwellinghouse?
For a house built after 1 July 1948 it is the house as originally built. For an older house it is the house as it stood on 1 July 1948. Every extension added since then counts towards the buildings total rather than being excluded, which is why a much-extended house has far less headroom than its owners expect.
Does selling garden land affect my permitted development rights?
Yes. The test applies to the curtilage as it exists when the works are carried out, so disposing of garden land permanently reduces the area in the denominator while the buildings already standing stay the same. Buildings that were lawful when built do not become unlawful, but the remaining allowance for new work shrinks.
Is the curtilage the same as my title boundary?
No. Curtilage is a question of fact and means the land serving the dwellinghouse as part and parcel of it, such as garden, yard and parking. A grazing paddock in the same ownership is generally outside it, so it cannot be added to the denominator, and land fenced off or sold drops out even if the deeds suggest otherwise.
Does a driveway count towards the 50 per cent?
Open hard surfacing and driveways are not ground covered by buildings for this test, and are dealt with separately under Class F. Boundary walls and fences fall under Part 2. Open decking is not a building, although raised platforms above 300 millimetres carry their own restriction within Classes A and E.
How do I prove the calculation?
Measure from a scaled survey and record the result in a short note with a plan showing the curtilage edged in red with its area, the original dwellinghouse hatched with its footprint, and every other building numbered in a schedule. Keep deeds showing any land disposal. A certificate under section 192 is the formal route to confirmation.
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.