Do I need planning permission for a rear extension?

If you're thinking about a rear extension, one of the first questions to work out is whether you need planning permission. The answer depends on a number of factors including the size of the extension, the type of property you live in, where it's located, and what's already been built before.

This post is a general introduction to how the planning system handles rear extensions in England. It's not advice on your specific project. Every property is different, and the only safe way to know what applies to you is to check with your local planning authority or speak to a planning professional before you commit to a design.

The three possible routes

In broad terms, a rear extension in England will fall into one of three categories.

1. Permitted development, no notification needed. Smaller extensions on most houses (not flats) can sometimes be built under what's called "permitted development." This is a national grant of planning permission for certain limited types of work. If your project genuinely falls within the rules, you don't need to submit a planning application, though it's often sensible to apply for a Lawful Development Certificate to confirm in writing that what you've built is lawful.

2. Permitted development, but prior approval required. Larger single-storey rear extensions can also fall under permitted development, but only via a process called "prior approval." This involves notifying your local planning authority before you build. They notify your neighbours, who have 21 days to object. If a neighbour objects on grounds the council considers reasonable, prior approval can be refused, and you would need to redesign or apply for full planning permission. A larger extension built without going through prior approval is not lawful, even if every other rule is met. This is the most common point of confusion.

3. Full planning permission required. Extensions that go beyond the permitted development limits, or are on properties where permitted development rights don't apply, need a full planning application.

The technical rules are set out in the Government's Permitted Development Rights for Householders Technical Guidance, which is the authoritative document on this topic. It's worth a read if you want to understand the system in detail.

Smaller rear extensions

For a single-storey rear extension on a typical house in England, permitted development rules generally allow projection from the rear wall of up to 4 metres for a detached house and 3 metres for a semi-detached or terraced house, subject to height limits and other conditions. Extensions within these dimensions, on properties that have full permitted development rights, often don't require a planning application at all.

That said, the rules also include conditions on overall height (typically 4 metres maximum), eaves height (typically 3 metres if the extension is within 2 metres of a boundary), materials (which generally need to be similar in appearance to the existing house), and how the extension relates to the rest of the building. Getting any of these wrong means the extension isn't permitted development, even if the depth is within limits.

Larger rear extensions and prior approval

Single-storey rear extensions of up to 8 metres on a detached house, or up to 6 metres on a semi-detached or terraced house, can also fall under permitted development, but only if prior approval is granted by the local planning authority first. This is sometimes called the "neighbour consultation scheme."

The process involves submitting a notification to the council with details of the proposed extension. The council writes to adjoining neighbours, who have 21 days to raise any objections. If no valid objections are received, prior approval is granted and the extension can be built. If valid objections are received, the council considers whether the impact on neighbours is acceptable. They can refuse prior approval, in which case a full planning application would be needed.

The thing to watch out for here is that prior approval is not automatic and not the same as planning permission. Building a 6 or 8 metre extension without first going through prior approval is not lawful, regardless of how well it otherwise fits the rules.

When permitted development doesn't apply

Permitted development rights don't apply to every property, and a full planning application is needed where they don't. The most common reasons rights might not apply or might be more limited include:

The property is a flat or maisonette
The property is in a conservation area, area of outstanding natural beauty, national park, or World Heritage site, where rights are usually tighter
The property is a listed building, in which case listed building consent is also required for external changes
The property has had its permitted development rights removed, either by an Article 4 direction or by a condition attached to a previous planning permission
A previous extension or alteration has already used up the permitted development allowance for the property

If any of these apply, the safe assumption is that planning permission is needed. A planning professional or the local planning authority can confirm.

Things that catch homeowners out

A few common misconceptions worth flagging:

Permitted development isn't universal. Two near-identical houses on the same street can have different rights if one is in a conservation area and the other isn't, or if one had its rights removed by an Article 4 direction or a previous planning condition.

Permitted development rights can be exhausted. If a previous owner extended the house, they may have used up some or all of the property's permitted development allowance. This isn't always obvious from the outside.

Building regulations are separate from planning permission. Even when planning permission isn't required, building regulations approval almost always is. They cover structural safety, insulation, fire safety, and so on. Planning and building regs are two different processes.

Prior approval is not planning permission. It's a notification process that allows certain larger extensions to proceed under permitted development. It's lighter-touch than a full application, but it's still a required step for the extensions it covers.

What to do before committing to a design

Three things worth checking before any design work begins:

Whether the property has full permitted development rights. Local councils' planning portals usually flag Article 4 directions, conservation area status, and listed building status. The Government's planning data service at data.planning.gov.uk is also a useful starting point.
The planning history of the property. Previous extensions, dormers, or outbuildings can affect what's still possible. Most councils' planning portals show planning history for an address.
Whether the specific design intended would actually fit within the rules. This is where the technical detail bites. The rules look simple on first read but include nuances around eaves height, ridge height, materials, and how the extension relates to the rest of the house. A planning professional can review a proposed design against the rules in a single conversation and flag anything that wouldn't qualify.

Where a planning professional helps

For straightforward small extensions on properties with clean planning history and no protected status, some homeowners do progress with limited professional input, often using the Lawful Development Certificate route to confirm lawfulness afterward.

For anything more involved, such as larger extensions needing prior approval, properties in conservation areas, listed buildings, complex sites, or designs that push the limits of what's allowed, getting professional advice early is usually a good investment. A short conversation with a planning consultant or architect can confirm whether an idea is feasible, identify which route applies, flag risks like neighbour objections or Article 4 restrictions, and outline what evidence the council will want to see.

Where to read more

The official starting points are:

Permitted Development Rights for Householders Technical Guidance on GOV.UK, which is the definitive technical reference
When is permission required? on GOV.UK, which gives a higher-level overview of the planning system
Your local council's planning portal, which is the place to check property-specific information

If you'd like to talk through your project with a vetted planning professional, you can sign up to PlanProve and post a project for free. Professionals on the platform can review your situation and tell you which route applies, what to expect, and what they'd need to put together a strong application if one's needed.