Most people ask this question after they’ve already decided what they want to build, which is the wrong order. Work through the checklist first and you’ll know within twenty minutes whether you need an application, and any architecture firm london homeowners approach should be running these same checks before drawing anything.
Nothing here replaces a proper assessment on your specific address. But it will tell you which conversation you’re about to have.
First: Is Your Property Even Eligible?
Start here, because four situations remove permitted development rights entirely and they catch people out before any dimension matters.
Flats and maisonettes have no permitted development rights for extensions. None. If you live in a flat, every alteration needs permission. This is the single most common misunderstanding we encounter in London, where flats make up a large share of the housing stock.
Listed buildings need listed building consent for most alterations, internal work included. Permitted development doesn’t apply in the way people assume.
Article 4 directions remove specified rights in defined areas. A council can apply one to a single street or across an entire borough. Hammersmith and Fulham, for instance, has a borough wide direction covering basement development, in force since April 2018.
Previously extended houses may have used the allowance already. The limits are measured against the original dwelling as built, or as it stood in 1948, not as it stands today. A previous owner’s extension counts against you.
If any of those four apply, stop. You need an application, and the permitted development route isn’t available regardless of what you build.
Rear Extensions: The Depth and Height Limits
Assuming your rights survive, here’s what GPDO 2015 Class A allows for a single storey rear extension.
| Limit | Attached house | Detached house |
| Standard depth | 3 metres | 4 metres |
| Larger Home Extension Scheme depth | 6 metres | 8 metres |
| Maximum height | 4 metres | 4 metres |
| Height within 2 metres of a boundary | 3 metres at eaves | 3 metres at eaves |
The larger depths need prior approval, which means the council notifies your neighbours and gives them 21 days to object. If someone does object, the council assesses the impact and can refuse. It isn’t automatic.
Two more limits apply regardless of depth. The extension can’t cover more than half the land around the original house, counting any existing outbuildings. And it can’t extend beyond the front elevation facing a highway.
Side Extensions: Narrower Rules Than People Expect
Side extensions are limited to single storey, a maximum height of 4 metres, and no wider than half the width of the original house.
That last one surprises people. On a 6 metre wide house, your side extension can be 3 metres wide at most.
In a conservation area, side extensions are not permitted development at all. That’s a designation level restriction, not a dimension one, so no amount of redesign brings it back within the rules.
Loft Conversions: The Volume Question
Class B covers roof additions. The allowances are 40 cubic metres of additional roof space for a terraced house and 50 cubic metres for a semi detached or detached house.
Volume is cumulative. Any previous roof addition counts, even one from decades ago.
Beyond volume, four conditions apply:
- No part of the addition can be higher than the highest part of the existing roof
- Nothing can extend beyond the plane of the existing roof slope facing a highway
- Any side facing window must be obscure glazed and non opening below 1.7 metres above floor level
- The addition must be set back at least 20 centimetres from the original eaves
The highway condition is what rules out front dormers on most London terraces. Rear dormers usually remain available.
Outbuildings: Often the Easiest Route
Class E covers garden buildings, home offices and garden rooms, and it’s frequently overlooked.
The building must be single storey, with a maximum height of 4 metres for a dual pitched roof or 3 metres otherwise, dropping to 2.5 metres if it sits within 2 metres of a boundary. Together with all other buildings it can’t cover more than half the land around the original house.
It also can’t be used as self contained living accommodation. A home office is fine. A separate annexe with its own kitchen and bathroom is not.
On a decent sized plot this route delivers usable space with none of the structural complexity of extending the house itself.
The Checks Nobody Does and Everyone Should
Four things, each of which takes minutes and each of which has ended projects that were already at drawing stage.
Check your planning history. Pull your address through the council’s planning portal. Previous applications, approvals and refusals are all public, and previous permissions sometimes carry conditions that removed permitted development rights on that specific property.
Check for an Article 4 direction on your exact address. Not your street, not your area. Some directions were drawn around specific house numbers, sometimes odd numbers only.
Check conservation area status separately. Conservation area designation and Article 4 are different controls that often overlap. A property can sit outside every conservation area and still have rights removed.
Check what previous owners built. Land Registry documents and old aerial imagery both help. If the house has been extended twice, your remaining allowance may be small or gone.
If You Are Within Permitted Development, Get It Confirmed
Permitted development means you don’t need permission. It doesn’t mean you have proof.
A lawful development certificate confirms in writing that the work was permitted development. It’s a formal application with a fee, and it’s worth it, because buyers’ solicitors ask for evidence and an unproven position on a title is a problem you’ll inherit at sale.
If You Need an Application, What Changes
A householder planning application means the council assesses the design rather than checking dimensions. Eight weeks is the target determination period, running from validation rather than from submission.
An incomplete drawing pack sits at validation before the clock starts, so the pack matters as much as the design.
The route being longer doesn’t make the extension less achievable. Plenty of schemes that fall outside permitted development get approved on their merits. It just means the design has to persuade somebody rather than simply comply with a table.
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