How close can an extension be to a boundary?

Planning rules don't impose a blanket gap between an extension and the boundary — plenty of side-return and terrace extensions are built hard against it. What changes near the boundary are the height limits: within 2 metres of any boundary, permitted development caps eaves height at 3 metres.

Two-storey extensions face a stricter test: under permitted development they must be at least 7 metres from the boundary opposite the rear wall. Closer than that means a full planning application, where the council will weigh the impact on your neighbour's light and outlook.

Separate from planning, the Party Wall etc. Act 1996 kicks in when you build astride or up to the boundary line, or excavate within 3 metres of a neighbour's structure below a certain depth. That means formal notice to your neighbour — a legal process, not a courtesy.

Building regulations add their own boundary consideration: external walls close to a boundary need fire-resisting construction, because the rules assume a future building could stand just over the line.

Practically, leaving even a small maintenance gap — for gutters, painting and repairs — avoids both overhang disputes and awkward access questions later. If your design goes to a full application, how the extension sits against the neighbouring property is exactly what the case officer will study.