Do I need planning permission for an extension in London?

Permitted development is a right, not a permission

Permitted development rights let you carry out certain building work without applying for planning permission. They come from national legislation rather than your local council, which is why the rules are broadly the same in Camden as in Croydon — and why the exceptions matter so much, because those are local.

The practical consequence is that you are not asking permission; you are relying on a right that either applies to your property or does not. If it does not and you build anyway, the council can require you to undo the work.

When permitted development does not apply

• Flats and maisonettes. Permitted development rights for extensions apply to houses only. There is no version of this that covers a flat.
• Listed buildings. Any external alteration will need listed building consent, and that is a separate consent from planning permission.
• Conservation areas. Side extensions and some cladding materials lose their permitted development status; rear extensions are restricted rather than banned.
• Article 4 directions. A council can remove permitted development rights for a defined area. Several London boroughs have done this street by street.
• Properties where rights were removed by a planning condition — common on newer estates.

Planning permission and Building Regulations are different things

This is the point that causes the most confusion. Planning permission is about whether you may build something and what it looks like. Building Regulations are about whether it is structurally sound, insulated, ventilated, drained and safe in a fire. An extension can be permitted development and still need full Building Regulations approval — and it almost always does.

Get a lawful development certificate anyway

If your extension is permitted development, you can apply for a Lawful Development Certificate confirming it. It is not compulsory. It is also the document a buyer's solicitor will ask for in five years' time, and without it the sale can stall while everyone tries to establish whether the work was lawful. It costs a fraction of a planning application.

The Party Wall etc. Act is a third, separate obligation

If you are building up to or astride a boundary, excavating within three metres of a neighbouring structure, or cutting into a shared wall, you have duties under the Party Wall etc. Act 1996 regardless of planning or Building Regulations. Notice periods run to two months for party wall work, so this belongs at the start of a programme rather than the end.