What size of extension can I build without planning permission?

So you're a homeowner in the UK, right? Single-storey rear extensions often fly under the radar without planning permission, thanks to Permitted Development rights. But it's not one-size-fits-all. Got a detached house? You can go 8 metres deep for that single-storey bump-out. Attached, like semi-detached or terraced? That drops to 6 metres. Height-wise, you're capped at 4 metres total, and if you're building within 2 metres of a boundary, eaves need to stay under 3 metres. Oh, and these rules? They're for houses, not flats or maisonettes. Plus there's other stuff—materials, boundary distances—that can trip you up.

Here's where it gets tricky. These allowances aren't automatic if you're building big. Under the larger home extension scheme, you've gotta give your local planning authority (LPA) a heads-up. If your extension is between 4 and 8 metres (or 3 to 6 for attached), you'll need a lawful development certificate or a prior approval application. And don't forget—Permitted Development vanishes in conservation areas, Areas of Outstanding Natural Beauty, or any designated land. Seriously, check with your council before you break ground. It'll save you headaches.

What are the exact size limits for a rear extension without planning permission?

Let's get specific. Under Permitted Development, rear extensions have clear boundaries. For a single-storey job: 8 metres deep max if your house is detached, 6 metres for anything else. Height tops out at 4 metres. And if you're within 2 metres of a boundary, eaves can't go above 3 metres. These measurements? They're from the original house's external walls—not any previous add-ons. The "original house" is defined as it stood on 1 July 1948, or as built if it came after. This is for the extension itself, not verandas, balconies, or raised platforms you might tack on.

Also—your extension can't swallow more than half the garden or curtilage around the original house. Materials should look similar to what's already there. Side extensions are an option too, but they're tighter: single-storey only, max 4 metres high, and no wider than half the original house's width. Front extensions? Yeah, those usually need planning permission. No way around it.

Can I build a two-storey extension without planning permission?

Short answer? No, you can't just throw up a two-storey extension under Permitted Development and call it a day. The rules are way stricter. A two-storey rear extension is allowed only if it's no more than 3 metres deep from the back wall of the original house. Plus, it's gotta be at least 7 metres from your rear boundary. Roof pitch needs to match the existing house as close as possible. And if there's a roof terrace or balcony involved, the extension can't be within 7 metres of a boundary. These limits apply to detached and attached houses alike, but prior approval from the LPA is always mandatory. Honestly, for any serious two-storey project, you're looking at full planning permission.

And heads up—two-storey extensions are a no-go on designated land like conservation areas or national parks without planning permission. If your house is listed, you'll need listed building consent for anything. My advice? Chat with your local planning authority. They'll give you the specifics for your property.

What about side and front extensions?

Side extensions can work under Permitted Development, but the limits are tight. A single-storey side extension can't exceed 4 metres in height and must be no wider than half the original house's width. Materials need to match what's there. If it's within 2 metres of a boundary, eaves height stays under 3 metres. And side extensions? They're not allowed on designated land without planning permission. Front extensions? Permitted Development mostly doesn't cover them. Anything to the front—like a porch or full extension—usually needs permission. A porch can sneak by without it if it's under 3 square metres floor area, no higher than 3 metres, and not within 2 metres of a boundary or highway.

Corner plots add another layer. Side extensions can't be built on the principal elevation or a side elevation facing a highway. Always double-check with your local planning authority for side and front extension rules. They vary more than you'd think.

What are the prior notification rules for larger extensions?

For bigger single-storey rear extensions—between 4 and 8 metres for detached houses, 3 to 6 for attached—you've got to apply for prior notification from your LPA. It's not full planning permission, but it's legally required. You'll submit scaled drawings, site plans, and a description of the work. The LPA then notifies neighbours (if you're within 4 metres of the boundary) and weighs the impact on amenity. They have 42 days to respond. If they don't, your extension is deemed approved. If they object? You can't proceed without full planning permission. This process is mandatory for these larger extensions, and skipping it can lead to enforcement action.

Prior notification also applies to two-storey extensions of any depth. For those, you might need a construction management plan if the extension is within 3 metres of a boundary. The LPA checks the impact on the area's character and neighbour privacy. Mess this up, and you could be forced to tear it down. Not fun.

What about extensions in conservation areas or on designated land?

If your property sits in a conservation area, Area of Outstanding Natural Beauty, National Park, or World Heritage Site, Permitted Development rights are heavily restricted. You can't build rear extensions of any size without planning permission. Same goes for listed buildings—any alteration or extension needs listed building consent. Even small stuff like porches or side extensions might require permission in conservation areas. The LPA will scrutinise the impact on the area's character and appearance. So check with them before planning anything in a designated zone. Sometimes you can build a small extension under Permitted Development if it's not visible from a highway, but that's rare.

Also, if your house is on a site of special scientific interest (SSSI) or an area of archaeological importance, any ground disturbance might need consent. Honestly, get a planning consultant or architect if you're in a designated area. It's worth it to avoid costly mistakes.

What is the maximum height for an extension without planning permission?

For a single-storey rear extension under Permitted Development, max height is 4 metres—that's the highest point of the roof. If you're within 2 metres of a boundary, eaves height can't exceed 3 metres. (Eaves are where the roof meets the wall, by the way.) Side extensions also cap at 4 metres. For two-storey extensions, height generally matches the existing house, but the ridge height can't exceed it. Roof pitch has to match too. These limits are strict and measured from ground level. Go over, and you'll need planning permission. Oh, and building regulations still apply—fire safety, structural stuff, all that.

Flat roofs max out at 4 metres, with drainage requirements. Pitched roofs? Ridge height stays under 4 metres. If you're adding dormers or other roof features, that might trigger planning permission. Always check height limits before you start. It's a pain to fix later.

Data Table: Permitted Development Extension Size Limits

Checklist: Is your extension likely to need planning permission?

• Is your house a flat, maisonette, or listed building? If yes, you likely need planning permission.
• Is your property in a conservation area, National Park, AONB, or World Heritage Site? If yes, you likely need permission.
• Is your extension more than 8 metres deep (detached) or 6 metres deep (attached)? If yes, you need permission.
• Is your extension taller than 4 metres? If yes, you need permission.
• Is the eaves height more than 3 metres within 2 metres of a boundary? If yes, you need permission.
• Is your extension a two-storey extension deeper than 3 metres? If yes, you need permission.
• Is your extension a side extension wider than half the house width? If yes, you need permission.
• Is your extension a front extension (other than a small porch)? If yes, you need permission.
• Will your extension cover more than half your garden area? If yes, you need permission.
• Are you building a balcony, veranda, or raised platform? If yes, you need permission.
• If you answered NO to all the above, your extension may be permitted under Permitted Development, but you must still check local conditions and apply for prior notification if the depth exceeds 4 metres (detached) or 3 metres (attached).

Frequently Asked Questions (FAQ)