Do I need planning permission for a rear extension in the UK?

Planning conditions that apply to all types of rear extensions in the UK

© Matt Gamble

Height

When building a single-storey rear extension, it cannot exceed 4m in height under permitted development. This is measured from the lowest ground level of your extension to the highest ridge on your roof. If you’d like to build a taller design, you’ll need to apply for planning permission from your local authority.

It’s worth noting that it’s unlikely you’ll be able to build your single-storey extension taller than 4m. Perhaps you’re looking for high ceilings or an A-Frame roof? If so, your designs will need to comply with all areas of planning as set in the legislation and/or SPDs.

Other height conditions include:

• Single-storey rear extensions can’t be taller than the highest part of your property’s existing roof. In other words, your extension can’t stretch higher than the existing uppermost eaves.
• If your extension comes within two metres of your property’s boundary (aka borders), the height at the eaves cannot exceed 3m.

Curtilage

In the early design stages of a new build, your proposed house is given a plot of land to sit on, which also belongs to you. Every part that isn’t taken up by your house is known as your curtilage. This usually makes up your front and back gardens and any side space you own (if you’re not in a terraced house).

Under permitted development, your extension plus any other structures can’t take up more than 50% of your curtilage. So, if you or any previous owners have already built an extension, it’ll be measured and included in the curtilage calculation, along with any sheds or outhouses. If they total over 50% of your curtilage, you’ll need to apply for planning permission.

Public highways

As per legislation, an UK extension cannot be built forward of the ‘principal elevation’ or, where it fronts a highway, the ‘side elevation’.

So, what does this mean? In short, the ‘principal elevation’ is a fancy way of saying ‘the front of your house’ and side elevation equally means ‘the side of your house.’ Furthermore, a highway means any public right of way, such as footpaths, roads, and of course, dual carriageways and motorways.

As rear extensions are tucked away at the back of your house, this rule doesn’t apply. It also means you don’t need to request planning permission if a footpath runs along the back of your property. Why? Because it’s not at the front or side of your house. The only time you’ll need planning permission is if a ‘highway’ is to the side or in front of your house. Simple.

Materials

When it comes to the materials you want to use for your extension, make sure they’re similar in appearance to the rest of your property. If they’re not, you’ll need to apply for planning permission. Examples include:

• Using a different material on your external walls, such as cladding instead of red bricks
• Installing casement uPVC windows instead of matching your timber-framed sash-windows
• Opting for slate roof shingles instead of your property’s current Marley concrete roof tiles

Misc works

If you want to include the following items in your rear extension, you need to apply for planning permission. None of the following fall under permitted development:

• Balconies, verandas, or raised platforms
• TV aerials or satellite dishes
• Chimney, flues, or soil and vent pipes
• Alterations to the existing roof of your house

Read our article on permitted development rights for more information.

Do I need planning permission to build a two-storey rear extension in the UK?

© Veronica Rodriguez

It’s tricky and complex to push a two-storey extension through permitted development, but it isn’t impossible, especially for those in detached and semi-detached houses. To guarantee success, you need to make sure there’s 7m between the rear of your extension and the rear boundary of your property (aka rear border/curtilage edge). If the rear edge of your extension is going to sit closer than that, you’ll need to apply for planning approval.

Your extension also can’t extend beyond the rear wall of your original house by more than 3m. So, even if a 4m extension still respects the 7m gap, you need to apply for planning permission to build it.

Other rules include:

• The pitch of your roof needs to match your existing house much as possible. This includes an upper-storey addition to an existing extension. If you want a steeper pitch to your existing roof, you’ll need to seek planning approval.
• If your two-storey rear extension is within 2m of a boundary (aka your borders or curtilage), your maximum eaves (not ridge) height needs to stay under 3m to fall under permitted development. As most two-storey extensions exceed 3m, most homeowners need to apply for planning permission when they want to build a two-storey rear extension within 2m of their borders.
  • NB: This means anyone living in a terraced house will almost definitely need to apply for planning permission to build a two-storey rear extension.
• Any upper-floor window located on the side of your two-storey extension must be obscure glazed and non-opening.
  • If the openable part of your window is more than 1.7 metres above the floor, this rule doesn’t apply.
• Any materials you use for your two-storey extension need to match the existing house as closely as possible.

Read our guide: Top 10 reasons planning permission is refused for more information.

Designated land and rear extensions

Designated land (Article 2(3)) includes national parks and the Broads, Areas of Outstanding Natural Beauty, conservation areas, and World Heritage Sites. Although permitted development still applies if you live in any of these areas, your extension options are more restricted. This also applies to green belt land.

1. You can’t use exterior cladding on your rear extension if you live on designated land.
2. You can’t build a side extension, nor add it to your rear extension if you live on designated land. However, it’s worth noting this depends on the local authority and how lax their rules are.
3. Rear extensions of more than one storey aren’t permitted on designated land. Similarly, this depends on the local authority and how lax their rules are.
4. Single-storey rear extensions must not extend beyond the rear wall of the original house by more than 4m if you live in a detached house or 3m for any other house.

Listed buildings

If you want to alter, extend or demolish a listed building in any way that affects its character as a building of special interest, you’ll need to acquire listed building consent from your local planning authority. This is mandatory whether planning permission is also needed or not.

Examples of extensions that didn’t require planning permission

Thanks to Resi's design expertise, these stunning rear extensions were built under permitted development. Would you like to see what we can do for you? Get in touch for a free consultation.

Elegant and simple rear extension under permitted development

We managed to design and build this elegant rear extension to meet permitted development. The homeowners applied for a Lawful Development Certificate to future-proof these works, which included bi-fold doors and plenty of extra space.

A spacious and bright rear extension in Lewisham

A beautiful rear extension in Lewisham with bi-fold doors, skylights, and a contemporary finish. This build was approved under prior approval and the larger home extension scheme. However, the homeowners opted to apply for a lawful development certificate to be on the safe side.

Extra kitchen space with a partial garage conversion

Our designers helped breathe new life into this London home with a complete floorplan transformation, a new open plan layout, and a partial garage conversion.

A rustic rear extension with Crittall style frames in Wandsworth

This stunning renovation applied for a dual application before it could undertake works: A larger home extension scheme under prior approval and a householder planning application.