How close can an extension be to a boundary?
An extension can be built right up to the boundary, but if it is within 2m of it the eaves height must not exceed 3m to stay within permitted development. Building on or near the boundary also triggers the Party Wall etc. Act 1996, which requires formal notice to your neighbour.
There is no minimum distance from a boundary for permitted development — you can build to the line. The constraint is height: within 2m of a boundary the eaves must be no more than 3m. That single rule is what shapes most side-return extensions in Fulham, because it dictates the roof form. It is also why so many side returns have a flat or shallow-pitched roof with rooflights rather than a pitched roof.
Building at or near the boundary brings in the Party Wall etc. Act 1996, which is separate from planning and cannot be skipped. Excavating within 3m of a neighbour's structure to a lower depth than their foundations, or building on the line of junction, both require notice — two months for a party wall notice, one month for a line of junction notice. In a Fulham terrace with neighbours on both sides, this applies almost every time.
Party wall matters are civil law, not council enforcement, which is why they are easy to overlook and expensive to ignore. Proceeding without a valid award gives your neighbour grounds for an injunction that can stop work mid-build. Budget £1,500 to £3,000 per adjoining owner for a surveyor, and start the process early — the notice periods run in parallel with design and planning, so served on time they cost you nothing in programme.
More on this in planning & permissions, or see all 42 questions.
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