Planning permission for an extension
London’s most common project. The permitted development envelope, what removes it, and the design tests an application faces.
Start with Class A — the permitted development envelope
Most single-storey rear extensions are built without a planning application under Class A of the householder permitted development rights. The limits are specific: depth beyond the rear wall, height caps, distance from boundaries, and the proportion of the garden you can cover.
If your design fits inside the envelope and no designation removes your rights, you may only need a lawful development certificate — worth having for resale even when permission isn’t required.
“This provides permitted development rights for the enlargement, improvement or other alteration of a house.”PD technical guidance, Class A · full text in the library → · official source →
What takes the shortcut away
Conservation areas restrict side and rear extensions of more than one storey and cladding; Article 4 directions can remove Class A rights entirely — common across London terraces. Flats have no Class A rights at all. If any of these apply at your address, the route is a full householder application.
The tests a full application faces
Design does the heavy lifting: how the extension sits against neighbouring rear walls, what your neighbours lose in light, materials against the parent building. National policy makes poor design a refusal ground in itself — and makes good design hard to refuse.
“Development that is not well designed should be refused, especially where it fails to reflect local design policies and government guidance on design , taking into account any local design guidance and supplementary planning documents which use visual tools such as design guides and codes.”NPPF paragraph 139 · full text in the library → · official source →
See it in a complete report
In our Clapham example, 211 of 262 decided extension applications within 1 km were approved — 81%.
General guidance ends here. Your property is specific.
The free check reads the designations on your exact property in seconds. The full report applies the policies above — and your council’s own Local Plan — to your project, with every quote verified against its source.