A designated area of special architectural or historic interest. Increased planning controls apply: Article 4 restrictions, demolition control, and design-led decisions.
A conservation area is an area designated under the Planning (Listed Buildings and Conservation Areas) Act 1990 for its special architectural or historic interest. Designation removes several permitted development rights, requires six weeks' notice before most tree work, and obliges the council to pay special attention to preserving or enhancing the area's character.
Article 2(3) removes three things outright: Class A side extensions, cladding, and Class B roof enlargement. Demolishing an unlisted building needs permission in its own right. London has 1,141 conservation areas, from 85 in Richmond upon Thames to four in Barking and Dagenham.
| What changes | Outside | Inside a conservation area |
|---|---|---|
| Class A side extension | half the original house width | not permitted development |
| Cladding the exterior | permitted | not permitted development |
| Class B dormer or hip-to-gable | 40 or 50 m³ | not permitted development |
| Rear extension depth | 3m, 6m under prior approval | 3m only, no prior approval route |
| Demolition of an unlisted building | no permission needed | permission required |
| Rooflights, Class C | permitted | restricted where facing a highway |
| Conservation areas in London | — | 1,141 |
Sources: the Town and Country Planning (General Permitted Development) (England) Order 2015, Article 2(3) and Schedule 2, Part 1, Classes A, B and C; section 196D of the Town and Country Planning Act 1990 on relevant demolition; conservation area counts from the planning.data.gov.uk register, retrieved 1 September 2026. Read at 1 September 2026.
Inside a conservation area you lose cladding, side extensions under Class A without permission, most roof extensions and dormers under Class B, chimney and flue installations on a wall or roof slope fronting a highway, and the larger rear extension prior approval route is restricted. Rear dormers in particular are the loss most homeowners feel.
You keep a single-storey rear extension within the standard 3 m or 4 m depth, rooflights that do not project materially beyond the roof plane, and internal alterations — a conservation area is not a listing, so the interior of an unlisted house is unaffected.
The statutory test is whether the proposal preserves or enhances the character or appearance of the area. In practice officers look at the street elevation first, then at materials, then at the rear where the impact on the wider group of buildings is assessed. A conservation area appraisal, published by the borough, usually sets out exactly what is valued.
That appraisal is the single most useful document to read before designing. It will name the prevailing roof form, the window pattern, boundary treatments and the negative features the council wants removed.
Article 4 directions are more common in conservation areas, tree work needs six weeks' written notice, and demolition of a building over 115 m³ or of a wall over 1 m high adjoining a highway needs planning permission for relevant demolition.
Conservation area in a Harrow context: London Borough of Harrow (Harrow Council) runs 29 conservation areas including the Harrow on the Hill Conservation Area and the Pinner Village Conservation Area, and there is no borough-wide Article 4 direction, so permitted development generally survives outside conservation areas. That decides how conservation area is handled: £548 and 8 weeks for a Harrow householder application, or £274 for the Lawful Development Certificate that records conservation area as lawful. The law behind conservation area sits in the Town and Country Planning Act 1990, the GPDO 2015 and the Building Regulations 2010.
| Legal basis | Planning (Listed Buildings and Conservation Areas) Act 1990 |
|---|---|
| Statutory test | Preserve or enhance character and appearance |
| Tree work notice | 6 weeks |
| Demolition threshold | 115 m³ building, or wall over 1 m by a highway |
| Rear dormer | Permitted development withdrawn |
| Application fee | £548 householder |
Yes. A single-storey rear extension within the usual depth limits generally remains permitted development.
Not by conservation area designation. Only listing protects interiors.
Usually only like for like, and an Article 4 direction may require an application.
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