A house extension can be a great addition to a property. Whether it's to improve saleability, increase its value, or add that awesome games room you've been dreaming about for years, there are plenty of reasons why homeowners want to add extensions to their homes.

For many types of work, a property owner needs planning approval from the local authority - they can't start building without it unless they want to risk future demolition of the project, or even face legal prosecution.

But if you don't want to get planning permission, there may be another way to move forward with your house extension. It just depends on its size, placement and other factors we'll outline below.

So, 'what size extension can I build without planning permission?' you ask us - well here's everything you need to know.

What is planning permission?

Planning permission is the approval homeowners need if they want to extend or make big changes to their property. You can apply for planning permission to the local planning authority, who will then either approve or reject your proposal.

Some development projects don't require planning permission, and can actually be carried out under permitted development rights.

What are permitted development rights?

This scheme was set up by the government to enable more homeowners to expand and build on their properties without planning permission. So long as the development falls under permitted development rules, home improvers don't have to go to the local planning authority to ensure their planning work is approved.

The benefits of building without planning permission

There are plenty of benefits to building without planning permission. One of the main benefits is reduced planning risks because you already have a clear idea of what is and isn't accepted as per permitted development rules. Less risk means a lower likelihood of hidden costs, having to reapply for planning permission, or even having your project refused.

Permitted development rules may also offer a good starting point for your project as you know what you can do within your boundary and have clear guidelines to help you get started.

The Larger Home Extension Scheme

In 2013, permitted development rights were extended under the Larger Home Extension Scheme. And before the scheme was dissolved in 2020, it allowed homeowners to build single storey rear extensions that were larger than permitted development rights allowed.

When the scheme was dissolved, some of the terms were added to permitted development rules, which meant homeowners could still build larger extensions, but would have to go through prior approval, rather than the larger home extension scheme.

Under prior approval, homeowners can extend a detached house up to eight metres, and six metres for other buildings.

The neighbour consultation scheme

The neighbour consultation scheme is part of the prior approval application. It informs neighbouring properties about any larger extensions that fall under permitted development rules. Neighbours then have the opportunity to express any objections or concerns they have about the project (for example if it devalues their property) after which the local authority will decide whether or not the impact of the project is too big for the surrounding properties. This will inform whether or not they grant planning approval.

The Party Wall Act

If there are disputes between neighbouring property owners, the Party Wall Act helps to resolve them. If you're unsure about the impact of your project, it's always best to seek legal advice so you know exactly what you're allowed to build.

What size extension can I build without planning permission?

If they meet certain criteria, single storey extensions such as conservatories and orangeries are covered by permitted development rights. These conditions include:

• The maximum extension for attached properties is up to three metres beyond the back wall of the original house.
• Detached property extensions can't extend beyond the back wall of the existing house by more than four metres.
• Single storey extensions can't be higher than four metres, and it mustn't extend past the rear wall by four metres for detached houses (and can't be over three metres for a semi detached or other buildings).
• A single storey extension width can't be wider than half of the original size of the house.
• For a two storey extension, the roof pitch must match the existing property, and first floor windows of any side elevations must be obscure glazed and non-opening (unless the open window is at least 1.7 metres from the floor).
• The roof and ridges extensions can't be higher than the original house and the eaves can't be higher than two metres within a three metre boundary.
• Extensions on the side or front of a property must be closer to the property than a public highway.

How big can a building be without planning permission?

You can build an extension without planning permission so long as it falls under permitted development rules. For example, you can add a house extension or conservatory up to eight metres if you live in a detached house (or up to six metres if you live in a semi detached house or terrace).

Conditions for building a house extension without a planning permission

No previous owners extended

If your house has been extended by previous owners (since 1st July 1948), you're not legally allowed to extend it without planning permission.

Designated land or conservation areas

If you live in a listed building or your property is built on designated land such as national parks or conservation areas (as well as other areas of outstanding natural beauty), you may have limited permitted development rights. You have to apply for listed building consent if you want to make any changes to a listed building, and your local council will only approve this if your proposed changes follow listed building regulations.

In some cases, there may not be any permitted development rights within designated land. It's always recommended to seek professional guidance if you want to build a house extension in this area.

Some single storey extension builds in conservation areas are covered by permitted development rights. If they're attached property extensions, they can't be built past the rear wall of the original house by more than three metres. If they're detached property extensions, they can't extend beyond the rear wall of the original property by more than four metres.

Houses versus flats

Flats aren't covered by permitted development; they only apply to houses. Flats and maisonettes require you to apply for planning permission.

Permitted development rules for different types of extension

There are different permitted development rules for different types of extension. Earlier we covered the basic development rules for certain types of extension, but here are some more outlined below.

For most extensions

For most property extensions, if you want to extend without planning permission, it cannot use more than half of the land surrounding the original property. The extension also mustn't exceed the tallest part of the existing house, and its eaves can't exceed the height of the existing eaves. The height of the extension's eaves mustn't exceed three metres if the extensions come within two metres of the boundary.

Additionally, you can't have forward or side extensions on a building too close to a public highway. Extensions can't include a chimney, microwave antenna or verandas, as well as any other alterations to the roof of the existing house. The building materials must also be similar to the materials used for the exterior of the original building.

A front, rear or side extension

Front extensions can't be more than one storey, mustn't front into the road and can't be more than three metres out from or more than half the width of the original house.

A side extension follows the same building regulations.

For rear extensions, a single storey extension can be up to four metres for a detached house, and three metres for semi detached and other houses. With prior approval, these extensions can increase to eight metres and six metres respectively. A double storey rear extension can go as far as three metres from the original house, as long as it's more than seven metres away from the rear wall.

Conservatory extensions

Conservatory extensions follow the same rules as house extensions. They can't be higher than the original house and can't occupy more than half the surrounding land. You can read more about conservatory extensions in one of our previous blogs!

Shed or garden room extensions

For shed or garden rooms, the outbuilding can't sit forward of the principal elevation. It can only be one storey with a maximum eaves height of 2.5 metres, and it can't be used for residential accommodation. You can find out more about planning permission for garden buildings here.

New fences or walls

New fences or walls can't be higher than one metre when next to a highway. Other fences or walls can't be higher than two metres, and you can't build them if you live in a listed building.

Swimming pools

You can build a swimming pool under permitted development rules if it doesn't take up more than half the garden curtilage.

Porch extensions

You can build a porch extension without planning permission if the porch isn't higher than three metres, and is no more than two metres close to any boundary next to a highway. The external ground area must also be below three metres squared.

Basement conversions

For basement conversions, you can build without planning permission as long as it's not a separate unit, you're not excavating to form a new basement, you're not significantly changing its use, and the external appearance isn't altered by the addition of a lightwell.

Loft conversions

If you want to build a loft conversion without planning permissions, it can't be more than 40 metres cubed, the dormer windows can't sit higher than the peak of the existing roof, and they also can't extend forward of the roof plane on the principle elevation. Read more about the planning rules for loft conversions here.

Garage conversions

As long as your project is internal and doesn't make your garage bigger, your garage conversion will probably fall under permitted development rules. However, if you're converting a detached garage you may have to apply for planning approval. Click here to read more about garage conversions.

Roof lights

The roof light mustn't project past 15cm from the roof's slope. It also can't extend forward on an elevation fronting a highway, and must be non-opening or more than 1.7 metres above the ground if placed on a side elevation. You'll need planning permission to install roof lights if you live in an area of outstanding natural beauty.

Solar panels

Solar panels under permitted development rules can't protrude more than 200mm past the plane, roof or wall. The highest part of the solar panel must also be lower than the highest part of the roof.

Knocking down internal walls

If you want to knock down an internal wall, you probably won't need planning permission so long as you don't live in a conservation area or listed building.

Replacing windows

As long as the new windows are similar to the originals, you don't need planning permission to replace them.

Lawful development certificates

Even if you don't have to seek full planning permission for your house extension, it's always recommended to get a lawful development certificate. It proves your work was lawful under permitted development rules, so didn't require planning approval. It's a really useful document to have if you want to sell your property in the future, as well as for general peace of mind.

Summary: Your dream extension awaits!

Great news! There are many circumstances in which you can build an extension without planning permission.

Different types of extension will have different planning rules, and you'll also have to be aware of any specific building regulations to your property or area. But so long as your project falls under the criteria we've talked about above, you'll be able to go ahead with your extension - no planning permission necessary!

House extensions can be a great way to ramp up the value of your property, increase future saleability or even just add more enjoyment to your home. If you are looking for ways to increase your property's value, start by checking out our Online Valuation Tool to see where your property sits in the market.