You've checked the depth limits, the height limits, and the boundary distances. Your extension or outbuilding meets every condition. But there's one more rule that catches homeowners out - especially on smaller plots where previous work has already eaten into the available space: all buildings on your plot must not cover more than 50% of the total curtilage area.
The curtilage is the land around your house that forms part of the property - typically the front garden, rear garden, side passage, and driveway. The 50% rule says that the total ground area covered by buildings within the curtilage - excluding the original house itself - must not exceed half the total curtilage area.
This is a cumulative calculation. It includes everything built since the house was first constructed (or since 1 July 1948 if built before then): rear extensions, side extensions, conservatories, garages, sheds, garden rooms, greenhouses, bike stores, and any other structure. If a previous owner built them, they still count. The allowance does not reset when the property is sold.
Every extension, shed, garage, and outbuilding counts. Built by you or a previous owner. The allowance doesn't reset.
Step 1: Work out your total curtilage area. This is the total land area of your property, measured within the legal boundaries. You can find this on your title plan from the Land Registry (£3 download). For a typical London terraced house on a plot of 5m x 25m, the total curtilage is 125 square metres.
Step 2: Subtract the footprint of the original house. "Original" means the house as first built, or as it stood on 1 July 1948. On our example terrace with an original footprint of 5m x 8m (40 square metres), the remaining curtilage area is 85 square metres. 50% of that is 42.5 square metres.
Step 3: Add up everything that's been built. A 15 square metre rear extension, a 6 square metre side return, a 4 square metre shed, and a 3 square metre bike store = 28 square metres. That leaves 14.5 square metres before you hit the 50% limit.
Step 4: Check your proposed project fits. If you want to add a 12 square metre garden room, the total becomes 40 square metres - within the 42.5 square metre limit. A 16 square metre garden room would take you to 44 square metres and exceed the rule.
If you're not sure how much of your curtilage is already used, the free eligibility check covers the key conditions for your project type.
Counts toward the 50%: rear extensions, side extensions, conservatories, garages (attached or detached), garden rooms, sheds, greenhouses, workshops, summer houses, bike stores, and any other roofed structure.
Does not count: the original house footprint, patios, decking under 300mm, driveways, hard standing, fences, walls, pergolas without a solid roof, and open-sided car ports (though the definition of "building" can be tested in borderline cases).
Demolished structures: if you demolish a shed or remove an old conservatory, that area is freed up. The 50% calculation is based on what exists at the time of the proposed development, not what has ever existed.
On a large detached property with a generous garden, the 50% rule is rarely an issue. But on a typical urban terraced or semi-detached house with a compact plot, it can become the binding constraint - particularly if the house has already been extended.
A terraced house with a rear extension, a side return extension, and a shed may already be close to 50% coverage. Adding a garden room could push it over the limit, even if the garden room itself meets every other Class E condition. This is why checking the cumulative coverage before committing to a design is essential. For more on how previous work affects your PD rights, see our previous extension guide.
Common mistakes that cost money
Forgetting to include small structures. Sheds, bike stores, and greenhouses all count. It's easy to overlook a 3 square metre shed, but on a tight plot it can make the difference.
Measuring from the extended house, not the original. The original house footprint is excluded from the calculation. Extensions are not. If you measure from the current footprint (including extensions), you'll underestimate the coverage.
Not knowing what a previous owner built. Check the planning history on your council's portal and look at historical aerial photos (Google Earth has a timeline feature) to identify structures that may have been added over the years.
Assuming the driveway counts. Hard surfaces like driveways and patios are not buildings and don't count toward the 50%. But a carport with a roof may count depending on its construction.
PD Assessment Tool
The 50% curtilage rule is just one of several conditions your project needs to meet. Our free eligibility check covers property type, designated area status, and the key conditions for extensions and outbuildings - so you know where you stand before you spend on design.
Free eligibility check. Full assessment £47.
Content verified against the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended), Class A paragraph A.1(b) and Class E paragraph E.1(b) of Part 1, Schedule 2 (legislation.gov.uk, revised version) and the government's technical guidance (September 2019). This page is for general information only and does not constitute legal advice.
April 2026