Advice
Flat or pitched roof on an extension? Pros, cons and the rules
The roof is one of the biggest design decisions on an extension. It affects how the extension looks from the garden, how much light gets in, what approvals you need, and how much it costs.
Pitched roofs
A pitched roof in tiles or slate to match the house usually looks the most natural, especially on older Wiltshire properties. Water runs off quickly, and a well-built pitched roof is very long-lasting. The trade-offs are more structure, more height, and the work of tying it neatly into the existing roof with valleys and lead flashings.
Flat roofs
A flat roof keeps the extension lower, suits modern designs and makes rooflights and lanterns easy to add. Modern GRP and EPDM systems are far more durable than old felt roofs. The catch is that a flat roof is not truly flat: it needs proper falls, a sound deck and careful detailing at upstands and outlets, or water sits on it.
Height and permitted development
Many single-storey rear extensions can be built without a planning application under permitted development rights, subject to limits on depth and height. Among other conditions, a single-storey extension within 2 metres of a boundary must not have eaves higher than 3 metres. The Planning Portal sets out the full rules, and our guide to planning permission and building regulations covers the Wiltshire process. A flat roof often makes it easier to stay inside the height limits.
Insulation and building regulations
Either roof must meet current building regulations for insulation and ventilation. On flat roofs a warm-deck build-up, with insulation above the deck, is the usual approach. Building control inspects the roof as part of the job.
Matching the house
On a period house in a conservation area, a pitched roof in matching materials may be expected. Check with Wiltshire Council planning early.