You're thinking about a two storey rear extension - an extra bedroom upstairs and more living space on the ground floor. Under permitted development, the maximum depth is 3 metres for all house types. But there's another rule that often becomes the binding constraint before the depth limit does: the rear wall of a two storey extension must be at least 7 metres from the rear boundary.
Take a typical London terraced house with a 10-metre garden. The original rear wall is at the back of the house. A 3-metre two storey extension would place the new rear wall 3 metres into the garden, leaving 7 metres to the rear boundary. That's exactly on the limit - it works, but there's no margin for error.
Now take the same house with an 8-metre garden. A 3-metre extension would leave only 5 metres to the boundary - less than the required 7 metres. Even though the extension meets the depth limit, the 7-metre boundary rule prevents it. Your maximum two storey depth would be 1 metre.
On a short garden, the 7 metre boundary rule - not the 3 metre depth limit - is what actually constrains your two storey extension.
The 7 metres is measured to the rear boundary of the curtilage - the legal property boundary, not the fence. Fences are often set inside the actual boundary, sometimes by 100-200mm. On a tight site where 7 metres is marginal, this matters. Download your title plan from the Land Registry for £3 to confirm exactly where the legal boundary sits.
If your rear boundary is not a straight line - for example, if it steps in or angles - measure from the closest point of the extension to the closest point of the boundary. The 7-metre rule must be met at every point across the width of the extension.
The 7-metre rule isn't the only condition. A two storey rear extension under PD must also have a roof pitch that matches the existing house as closely as practicable. The eaves and ridge must not exceed the existing house. And on designated land (conservation areas, National Parks, AONBs), two storey rear extensions are not permitted development at all - you need planning permission regardless of the garden length.
There is no prior approval route for two storey extensions. The prior approval scheme only applies to single storey extensions. If your two storey extension exceeds the 3-metre depth or can't meet the 7-metre boundary rule, you need a full householder planning application (£548).
For all extension conditions including single storey rules, see our extensions guide.
Not sure whether your garden is long enough? The free eligibility check covers property type, designated area status, and the key conditions for extensions.
Common mistakes that cost money
Measuring from the fence, not the legal boundary. On a tight garden, the difference between the fence line and the legal boundary can push you below 7 metres. Always check the title plan.
Not accounting for an existing single storey extension. Depth is measured from the original rear wall. If there's already a ground floor extension, the two storey element must still be within 3 metres of the original wall and 7 metres from the boundary.
Assuming the prior approval route works for two storeys. It doesn't. The larger home extension scheme (6m/8m) is for single storey only. A two storey extension has a hard 3-metre limit with no exceptions under PD. See our prior approval guide.
PD Assessment Tool
A professional permitted development assessment from a planning consultant typically costs £400-600. This tool checks your specific project against every Class A condition - including the 7-metre boundary rule - and produces a formal document you can share with your builder or council.
Free eligibility check. Full assessment £47.
Content verified against the Town and Country Planning (General Permitted Development) (England) Order 2015 (as amended), Class A of Part 1, Schedule 2, paragraph A.1(h) (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