Do I need planning permission for a loft conversion?
A rear dormer within permitted development limits — 40m³ of added volume on a terraced house, 50m³ on a semi or detached, nothing forward of the front roof plane — usually does not need planning permission. A mansard, a front dormer, or any conversion in a conservation area normally does.
Permitted development allows a loft conversion adding up to 40 cubic metres of volume on a terraced house or 50 cubic metres on a semi-detached or detached house. The extension must not go beyond the plane of the existing roof slope on the principal elevation — in practice, no dormers on the front — and must not be higher than the highest part of the existing roof. Side-facing windows must be obscure-glazed and non-opening below 1.7m.
The 40m³ allowance is more restrictive than it sounds on a Fulham terrace. A full-width rear dormer on a typical terrace can approach or exceed it, particularly if the house has been extended before, since previous additions count towards the total. This is one of the most common reasons a Fulham loft design has to be adjusted or submitted for full planning permission.
In a conservation area — including the Peterborough Estate — permitted development rights for roof alterations are restricted, and a dormer visible from the street will generally be resisted. A mansard almost always needs full planning permission anywhere. Whichever route applies, building regulations approval is required in every case, and that is not optional.
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