Do you need planning permission for an extension?

The short answer

In England a house comes with a set of permitted development rights: things you may build without applying for planning permission, provided they stay within limits set out in the General Permitted Development Order. Class A of that order covers extensions. If your extension stays inside the limits below, and none of the exceptions apply, you do not need to apply. If it goes over any one of them, you do.

Two things first. These rights apply to houses, not to flats or maisonettes. And they can be removed: by a condition on an earlier planning permission, or by an Article 4 direction that the council has placed on your area. If either applies, everything needs an application. Wales, Scotland and Northern Ireland have their own rules.

Single-storey rear extensions

• Up to 3 metres beyond the original rear wall on a semi-detached or terraced house, and up to 4 metres on a detached house.
• Larger, up to 6 metres on an attached house and 8 metres on a detached one, under the larger home extension scheme. This needs prior approval: you notify the council, the council consults the neighbours, and it confirms within the statutory period whether you may proceed.
• No higher than 4 metres.
• Where the extension comes within 2 metres of a boundary, the eaves may be no higher than 3 metres.

Side extensions

• Single storey only.
• No higher than 4 metres.
• No wider than half the width of the original house.

Two-storey extensions

• No more than 3 metres beyond the original rear wall.
• At least 7 metres from the boundary opposite the rear wall.
• The roof pitch should match the existing house as far as practicable.
• Upper-floor side windows must be obscure-glazed and non-opening, unless the opening part is more than 1.7 metres above the floor.

Rules that apply to all of them

• Nothing forward of the principal elevation, or of a side elevation that fronts a highway.
• No higher than the highest part of the existing roof, and no higher at the eaves than the existing eaves.
• Materials similar in appearance to the existing house.
• Additions, including earlier ones, may cover no more than half the land around the original house.
• No verandas, balconies or raised platforms.
• On Article 2(3) land, which includes conservation areas, national parks and areas of outstanding natural beauty, no cladding, and any side or two-storey rear extension needs full permission.

The phrase original house matters. It means the house as it was first built, or as it stood on 1 July 1948 if it is older than that. An extension added by a previous owner has already used up part of the allowance.

When you must apply

Over any of the limits above, on a flat, on a listed building (which also needs listed building consent), where an Article 4 direction or a planning condition has removed the rights, or where the design departs from the conditions on materials, height or position. A wraparound that joins a rear and a side extension is usually treated as one extension against the side rules, and usually needs an application.

Get it in writing either way

Even where the work is permitted development, apply to the council for a lawful development certificate. It is a formal confirmation that the extension is lawful, based on the drawings you submit. When the house is sold, the buyer's solicitor will ask for it.

What planning permission does not cover

Whether or not you need permission, the extension needs building regulations approval: the structure, the foundations, the insulation, the drainage, the electrics and the fire safety are all checked by a building control body. The difference between the two approvals is set out in planning permission versus building regulations. If the new walls or foundations come near a neighbour's building, the Party Wall Act applies as well.

When the studio draws an extension, the permitted development check is the first thing on the drawing, and the same set of drawings is then prepared for building control, so the approvals run in order rather than in parallel. The order is on the process page.