Flat Roof vs Pitched Roof: Which Is Right for Your Extension?

If you're planning a single-storey extension, one of the first decisions is what goes on top. Flat and pitched roofs both work well, but they suit different situations, and "flat roofs always leak" is outdated advice based on materials from decades ago, not what's actually fitted today.

How flat roofs work today

Modern flat roofs use single-ply rubber membrane (EPDM), not the old bitumen felt that gave flat roofing its bad reputation. Properly installed, EPDM typically lasts 20 to 25 years with very little maintenance. Flat roofs also tend to be quicker and less disruptive to install, since there's no need to match complex roofline junctions with the existing house.

How pitched roofs work for extensions

A pitched roof on an extension is usually chosen to match the existing roofline, particularly where the extension is highly visible from the street or where planning guidance favours a consistent roofline. It can also make more sense structurally on larger extensions, or where you want usable loft space above the new room.

Cost and practical differences

Upfront cost tends to favour flat roofs, since they're usually cheaper to install for a given size due to simpler construction. Flat roofs also give you more headroom relative to the wall height, which is useful on extensions with height restrictions. Pitched roofs, on the other hand, blend more naturally with an existing pitched-roof house, so they tend to win on visual match. Natural light is easier to add either way: flat roofs make it easy to add a roof lantern or large flat rooflight, while pitched roofs suit Velux-style roof windows instead.

Planning permission and building regulations

Most single-storey rear extensions fall under permitted development, but rules vary by property, conservation area status, and local authority. Roof height and pitch can affect whether an extension needs full planning permission. It's always worth checking with your local planning department or an architect before finalising the design, regardless of which roof type you choose.

Either way, get it surveyed

We're happy to talk through both options for your specific extension and give you a straight recommendation, not just whichever is easier for us to fit.