Do I Need Planning Permission for an Extension?

Applies to England. Wales, Scotland and Northern Ireland have separate rules.

The size limits

For a single-storey rear extension:

Within the standard limit you need tell nobody. Beyond it, the prior approval route means notifying the council, who consult your neighbours and consider the impact. It is lighter-touch than a full application and generally reliable where there are no substantial objections, but starting a 6- or 8-metre extension without it makes the work unlawful, not merely unpermitted, and the council can require demolition back to the permitted depth.

For a side extension: single storey only, maximum eaves height 3 metres, and no wider than half the width of the original house.

For a two-storey rear extension: full planning permission, always. It also cannot come within 7 metres of the rear boundary.

The conditions people breach

Size is only the first test. All of these apply as well:

• Maximum height 4 metres for a single-storey extension, with eaves no higher than the existing house.
• Eaves within 2 metres of a boundary must not exceed 3 metres. Contractors routinely build to the full 4 metres right up to the line, which breaches this even where the overall height is fine.
• No more than half the land around the original house may be covered. Sheds, garages and earlier extensions all count.
• Materials similar in appearance to the existing house.
• No verandas, balconies or raised platforms above 300mm. Roof terraces and Juliet balconies take you outside permitted development.
• Upper-floor side windows must be obscure-glazed and non-opening below 1.7 metres from floor level.
• Nothing forward of the front elevation facing a highway.

Measure from the original house

This deserves its own section because it causes more enforcement problems than anything else.

The limits are measured from the original rear wall, the house as first built, or as it stood in July 1948. Not from an extension a previous owner added. If your house already has a 2-metre extension, a further 3 metres does not give you 3 metres of permitted development. It gives you nothing.

Confirm the original footprint before anyone draws anything.

Where permitted development does not apply at all

• Flats and maisonettes. No permitted development rights for extensions, including ground-floor flats.
• Listed buildings, where listed building consent is needed as well.
• Conservation areas and other designated land, where rights are restricted.
• Article 4 directions, where the council has removed rights across an area, common in parts of London and in some estates.
• Properties where rights were removed by a planning condition, often on newer developments.

Check with your local planning authority. Do not assume your neighbour's project sets a precedent.

Be careful what you read

Several widely-published guides describe proposed changes to permitted development as though they were in force, quoting larger rear extension limits than the law currently allows. As at the time of writing, the standard limits remain 3 metres and 4 metres.

If a builder or a website tells you a bigger extension is permitted development, ask them to point at the current rules. Building on the strength of a proposal that has not passed is how people end up demolishing work they have paid for.

Proving it

Where work is permitted development, you can apply for a lawful development certificate confirming it. It is not compulsory. It is also cheap relative to the risk, and it answers the buyer's solicitor at sale without an argument.