Planning permission for extensions: Find out if you need consent for your project

Blockwork being built up for an extension on a semi detached house with a yellow cement mixer in the foreground
Planning permission for extensions isn't as simple as it making sure your design falls within permitted development as there are other factors that will need considering (Image credit: Getty)

Working out if you need planning permission for extensions is fundamentally about assessing whether permitted development rules apply. Permitted development allows certain works to be undertaken on your home without needing to seek planning permission first. 

There are other factors that need considering to be able to do this, including looking at whether you home is in a conservation area, if it is listed, or if the original house has already been extended. Some local authorities may have withdrawn permitted development rights in your area too, so you need to be on the lookout for that.

And with plenty of homeowners now preferring to extend rather than move, there's never been a better time to get a firm grasp of the rules.

Our planning expert Simon Rix takes on some of the more frequent questions he often hears when it comes to planning permission for extensions. 

Planning permission for extensions: A quick guide to permitted development

Permitted development enables you to go ahead with a project without needing to apply for planning permission, as long as it fulfils certain criteria.

Permitted development – or PD – usually would not apply to flats, maisonettes, some other buildings and certain areas where councils have withdrawn permitted development, in what is called an 'Article 4 directive'. You would always need to double check whether this applies in your area.

Broadly speaking, projects that might fall within permitted development include porches, small extensions, as well as loft and garage conversions. Updates to PD rights in recent years, include making it easier to convert a commercial building into a home, as well some two-storey extensions.

Our extensive guide on permitted development is well worth a look, if it isn't just extensions you are hoping to add to your home. 

It's important to remember that even if your project does fall within permitted development, you still need to make sure your extension is built to the current building regulations with building control sign off.

open plan kitchen dining view through sliding doors

Even if you extend under permitted development, you'll still need building control sign off (Image credit: Mark Ashbee)

Do I need planning permission for my extension?

Permitted development rules can be a rather dense minefield to navigate. While one extension might easily fall within the rules, another similar extension on a different property might not. There are also size limitations that may apply.

As such, the best way to answer whether or not planning permission might be required for an extension or not, is with some examples.

1. Yes your project will need need planning permission if it's a double storey extension to the side

The answer to this question is definitely yes if you want to add a double storey extension onto your property on the side.

2. No it doesn't necessarily need planning permission, if it's single-storey and under half the width of the existing dwelling

If no restrictions apply that withdraw PD rights, a homeowner should be able to build a single-storey extension to the side of their property under permitted development for 'Class A' improvements. This is under for extensions up to half the width of the existing dwelling.

3. Possibly, if your home is in a special area

Even a single storey side extension may require planning consent if the house is within a National Park, the Broads, an area of outstanding natural beauty, a conservation area, or if it’s within a World Heritage Site. The rules on what is allowed do however vary from council to council so its worth checking with them in advance.

4. Yes, if your council has withdrawn PD rights

You may also live in an area that is covered by what’s called an 'Article 4 Direction' that limits or removes your right to build an extension without planning consent. You will need to put in a planning application if that’s the case. If you do, it's worth checking out our guide to the secrets to planning success.

5. No, if it's a single-storey rear extension up to 4 metres in length

This PD right only applies to detached dwellings but allows a single-storey rear extension up to 4 metres. If your home is a semi or terraced property, then you can still add a rear single-storey extension under 'Class 4' permitted development rights but this instead must be 3 metres or less. 

In some situations you might even be able to add a 3 metre two-storey rear extension. Again, all of this is true as long as PD restrictions aren't in place.

6. Yes, if your extension goes beyond a side elevation and fronts a highway

If your house extension idea stretches beyond a side elevation of your house and if that side elevation fronts onto a highway, you'll also need planning permission. 

Basically, if your extension protrudes beyond the principal elevation of your house, permitted development rules would no longer apply. The principal elevation of your house may be obvious, but often this can be a grey area, so you should think about getting some professional advice on this if you are in any doubt. 

7. No, if you get prior approval for an 8 metre single-storey rear extension

There was an update to 'Class A' PD rights on 30 May 2019, that means homeowners can build extensions of up to 8 metres on the original detached house under permitted development if they successfully seek prior notification. The same is broadly true on semi and terraced house for extensions up to 6 metres. 

8. Yes, if your home is a converted house or created through a PD right

You will also need planning consent if yours is a flat or a maisonette, if it’s a converted house or a house that was originally created through a PD right, either as a change of use or a completely new dwelling, or if it was consented with a planning condition that specifically prevents the type of extension you want.

9. Yes, if your extension results in more than half your garden being developed

A planning application will also be needed if your extension would result in more than half the area of land around your 'original house' being covered by extensions or other buildings. 

Although you may not have built an extension to the house, a previous owner may have done so, so it’s important to work out what the 'original house' was, because the definition is the house as it was first built, or as it stood on 1 July 1948, if it was built prior to that.

10. Likely, if your house is listed

You will almost certainly need listed building consent if your home is listed. Whether or not you also require planning permission will depend on your local council.

11. Yes, if it includes a balcony

To benefit from permitted development rights, an extension cannot include any verandas or balconies, except Juliet balconies. 

An extension must also not include any raised platforms, TV aerials, satellite dishes, chimneys, flues or soil/vent pipes or any alteration to the roof of the existing house if you are carrying out the work under permitted development rights.

The front view of a semi-detached home that is having a blockwork extension added to the side

A double storey side extension would require planning permission (Image credit: Getty)

How far can you extend without planning permission?

For a single-storey extension to fall under PD, you can't extend beyond the rear wall of the ‘original house’ by more than four metres if you live in a detached house or by more than three metres for any other house. 

That said, prior approval now enables longer extensions to be built without a full planning application.

For a single storey extension, where you meet all the other permitted development requirements, you can extend back eight metres beyond the 'original house' if it’s a detached house if prior approval is sought and approved.

For houses that aren't detached, you can go back six metres if you apply for prior approval.

What is "Prior Approval"?

Prior approval is a bit like a mini-planning application process, with fewer details required to be submitted and fewer reasons why the council could say no. 

For an extension of more than one storey, you must not extend beyond the rear wall of the 'original house' by more than three metres or be within seven metres of any 'boundary' opposite the rear wall of the house without making a planning application. 

A 'boundary' may not be the boundary of your landholding. It means the boundary of the curtilage of your house, so a wall between a garden that immediately surrounds your house and a field that you also own beyond may be what counts as the boundary.

In terms of width, side extensions can only be up to half the width of the 'original house' to avoid a planning application.

If you are curious, here are some other things you can do without planning permission.

corten clad modern extension to terrace house

You may need prior approval for your plans, even if you don't need planning permission. Contemporary-style extensions are likely to require planning. (Image credit: Simon Maxwell)

How high can you extend without planning permission?

A planning application will be needed if your extension will be higher than the highest part of the existing roof, or higher at the eaves than the existing eaves. 

Also, if your extension would come within two metres of the ‘boundary’, the eaves height cannot be more than three metres for the project to be covered by PD rights.

Permitted development, if applicable, would only cover additions to the roof under 'Class B' as long as the additional volume created is no larger than 50m3 for detached homes and 40m3 for other homes.

Are there any restrictions on my extension roof pitch?

If the project involves more than one storey then to use permitted development rights, the roof pitch must match existing house as far as practicable. This also applies to any upper storey built on an existing extension. 

The materials used in any exterior work must be of a similar appearance to those on the exterior of the existing house too.

To avoid needing planning permission for extensions, you must ensure any upper-floor window located in a side elevation is obscure-glazed and non-opening, unless the openable part is more than 1.7 metres above the floor.

A window is usually not permitted either in a roof slope on a side elevation of an upper storey.

How much will planning permission cost?

The fee you will need to pay your local planning authority for this type of planning application is likely to be £206 in England or £190 in Wales.

Additional costs to consider include hiring a planning consultant, architect, and surveys. Collectively, these costs can be substantial depending on your plot and the complexity of your project. For more information check out how much does an extension cost in our guide.

brick and glass rear extension to terrace house with view to kitchen

There are plenty of costs to consider when it comes to extensions, even if you can build under permitted develoment (Image credit: Andrew Beasley)

Do I need an architect or can I apply myself?

You don’t need an architect or designer to create your plans (although it's recommended), but it’s usually a good idea to at least get an architectural designer to look over what you want to do. If you need some ideas, take a look at our modern extension ideas or our guide on semi detached house extensions.

If you go down this route it might also be best to discuss your plans with a planning consultant to see if what you want to do will likely be approved under local and national planning policy.

You can apply for planning permission through your local planning authority, normally your district or unitary council. This will be done via the planning portal.

It can be quite complicated so it’s usually best to get a professional planning consultant to help you.

How long will it take before planning is granted?

Planning permission should be granted within eight weeks, but many councils are missing this target and planning permission can take several months depending on the complexity of your project.

If listed building consent is required this is a separate application process and can take several months as well as a fair amount of back and forth before you get approval.

What happens if my extension plans are rejected?

Usually the council will get in touch to advise you of possible amendments to your plans before issuing a refusal. You then have the ability to withdraw, change or tweak your application and then resubmit it.

If the amendments are unworkable or you believe you may win an appeal, you can also do this. Take a look at our planning permission refused guide for more information.

Simon Rix

Simon Rix is a professional planning consultant, who began his career working in local government in the 1990s. He was a council officer and later an elected councillor, so he knows how the planning system works from both sides. He went on to set up Planix.UK Planning Consultants Ltd; a consultancy company that advises self builders, home extenders and those taking on small to medium-sized building projects on planning permission.