Roof design must address weather protection, loads and energy performance as well as planning appearance. [1] [2]
Check the planning envelope first
For a single-storey extension under Class A, the overall height limit is 4m, with an eaves limit of 3m where the extension is within 2m of a boundary. Other limits related to the existing house also apply. A roof that fits inside a sketch line may not fit once the construction thickness has been resolved. [3]
The roof-pitch condition is particularly important for enlargements of more than one storey and upper storeys added to existing extensions: so far as practicable, the pitch must match that of the original house where Class A applies. It is not a general rule that every single-storey addition must have a pitched roof. [3]
For a planning application, check the local design guide and any heritage requirements. Brent's guidance distinguishes different extension forms; Historic England advises assessing the relationship of an addition to the existing building. [4] [5]
Compare a flat roof properly
“Flat” does not mean that rainwater can be left without a designed drainage route. Ask the drawings to show falls, outlets, upstands and the junction with the existing wall. The covering and supporting construction need to work together. [1]
The insulation arrangement also matters. A warm-roof and a cold-roof design place insulation differently and have different ventilation and moisture considerations. Do not approve a roof solely on the basis of a material name or a nominal insulation thickness. [6]
Include rooflights and any raised roof features in the planning-height check. Their openings and structural trimming also need to be coordinated with the roof design. [7]
Compare a pitched roof properly
Show the ridge, eaves and abutment against the existing house in section. Ask how the roof meets upstairs windows and the original wall, and how its covering is suited to the selected pitch. New roof construction needs an appropriate structural and weatherproofing design. [1]
A vaulted ceiling should be drawn with the insulation, structure and rooflight details included. Otherwise, the apparent internal height may not represent the buildable room.
Decide on maintenance and the finished room
Ask both options the same questions: how will gutters and outlets be reached, where will water discharge, which details are concealed, and how will the retained house connect to the new space? Request these answers in the technical drawings rather than leaving them as assumptions in a builder's quotation.
For work proceeding from 24 March 2027, review the new Approved Document L edition and the transitional arrangements. The 2026 document contains extension guidance for existing homes; the new-home Future Homes Standard should not be confused with every requirement affecting an extension. [8] [9]