You're designing a rear extension on a narrow plot. The house is close to the boundary - most terraces and many semis are - and you want to know how tall the extension can be before it breaches permitted development. This is the rule that catches people on tight sites.
3 metres. If any part of your extension is within 2 metres of a boundary, the eaves height of the extension cannot exceed 3 metres. This is separate from the overall 4-metre single storey height limit, and it's a different rule from the 2.5-metre total height limit that applies to outbuildings.
Extensions have two height conditions that both must be met.
The eaves are the point where the roof meets the top of the wall - where the guttering sits. On a flat-roofed extension, the eaves and the overall height are effectively the same thing, so the 3-metre eaves limit also becomes a 3-metre total height limit when you're within 2 metres of a boundary.
On a pitched-roof extension, you could have 3-metre eaves with a roof that rises to nearly 4 metres at the ridge - but only if the extension is close enough to the boundary for the eaves rule to apply and far enough for the ridge to stay under 4 metres. In practice, on narrow plots, the 3-metre eaves limit is almost always the binding constraint.
On a typical terraced house, the extension is almost always within 2 metres of the boundary. The 3 metre eaves limit applies to most real-world projects.
Outbuildings (Class E) have a different rule: within 2 metres of a boundary, the total height is capped at 2.5 metres. Extensions (Class A) cap the eaves at 3 metres within the same distance. Confusing these two rules is one of the most common mistakes homeowners make.
For the outbuilding boundary rules, see our outbuilding height near boundary guide.
The government's technical guidance defines eaves as the point at which the external wall of the building meets the roof covering. In simple terms, it's the bottom edge of the roof - the gutter line. The measurement is from the ground level directly below the eaves to the underside of the roof where it meets the wall.
On a parapet wall (where the wall continues above the roof line), the eaves height is measured to the top of the parapet. This can push you over the 3-metre limit if you haven't accounted for it in the design.
If you're designing an extension on a tight plot, the free eligibility check covers boundary distances and height limits.
Three metres of eaves height gives you roughly 2.4-2.5 metres of internal ceiling height once you account for the floor build-up and the roof structure. That's comfortable for a single storey room - standard ceiling height in most houses is about 2.4 metres. For a flat-roofed extension right up to the boundary, 3 metres is workable. For a pitched-roof extension, the lower eaves give you a sloping ceiling at the edges, but full height in the centre of the room.
Where it becomes difficult is when the ground level differs between your garden and the neighbour's. If their garden is lower, the 3-metre eaves might feel much taller from their side. And if your garden is lower, you effectively lose height because you're measuring from the higher adjacent ground level. On sloping sites, this rule can reduce your usable ceiling height significantly.
Common mistakes that cost money
Confusing extension and outbuilding rules. Extensions within 2m: 3m eaves limit. Outbuildings within 2m: 2.5m total height limit. These are different rules under different classes.
Not measuring to the legal boundary. The 2-metre distance is from the boundary of the curtilage, not from the fence. Download your title plan from the Land Registry for £3 to confirm the boundary position.
Forgetting parapet walls count. If your extension design includes a parapet wall above the roof line, the eaves height is measured to the top of the parapet, not the gutter.
PD Assessment Tool
A professional permitted development assessment from a planning consultant typically costs £400-600 and takes 2-3 weeks. This tool checks your specific project against every Class A condition - including height limits, boundary distances, and eaves restrictions - and produces a formal document you can share with your builder or your council.
The first step checks whether your property is eligible - about two minutes, completely free. If your property qualifies, the full project assessment is £47.
Content verified against the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended), Class A of Part 1, Schedule 2 (legislation.gov.uk, revised version) and the government's technical guidance (September 2019). SI 2026/313 confirmed no changes to Class A. This page is for general information only and does not constitute legal advice.
April 2026