Both roof types are used constantly on single-storey extensions, and both are entirely legitimate choices — this is not a case of one being inherently better built than the other. The deciding factor is almost always what sits directly above the extension.
The headroom constraint
Where an extension sits under an existing first-floor window, a pitched roof at a sensible angle often rises high enough to block light to that window or breach the sill line. A flat or very low-pitch roof avoids this because it keeps the whole structure below the sill.
Where there is no window above, or the extension runs out from a single-storey part of the house, a pitched roof has room to work and can tie into the existing roofline more naturally.
Rooflights and internal ceiling shape
Flat roofs make rooflights simple to specify and position anywhere across the deck, which is why so many kitchen extensions with flat roofs feature a run of rooflights along the ridge line. A pitched roof can still take rooflights, but they follow the slope and read differently inside the room.
Vaulted or sloped ceilings under a pitched roof can make a room feel taller and more architectural, which flat roofs cannot replicate without a lantern or raised section.
Service life and maintenance
A well-specified flat roof covering — EPDM or a similar single-ply membrane laid to correct falls — has a long, predictable service life, but flat roofs generally need more careful detailing at upstands and outlets to avoid ponding.
A pitched roof covered in tile or slate is a more conventional build-up that most roofers are familiar with, and rainwater disposal is simpler because gravity does more of the work.
Cost drivers beyond the basic structure
On a flat roof, cost is driven mainly by insulation thickness, the quality of the membrane specified and how many upstands, outlets and rooflight kerbs need detailing — each junction is a place labour concentrates.
On a pitched roof, cost tracks the covering material, the complexity of any valleys or hips created where the new roof meets the existing one, and whether trussed or cut-roof construction is used to span the extension.
When each is clearly the wrong choice
A pitched roof is the wrong choice where it would sit above the sill of an existing first-floor window, since raising the extension's roofline that high is rarely acceptable to a planning authority even where the extension footprint itself would be permitted.
A flat roof is the wrong choice where the household specifically wants a vaulted or raised ceiling inside the extension, or where the existing house roofline is steeply pitched and a flat addition would look visually disconnected from it.
Common questions
Does a flat roof always mean lower quality?+
No. A flat roof built with proper falls, insulation above the deck and a good single-ply membrane performs reliably. Reputation problems come from old felt roofs laid without adequate falls, not from flat roofs as a concept.
Can I mix the two on one extension?+
Yes — a common solution is a flat section under an existing window with a pitched or vaulted section elsewhere on the same extension, giving light where it is needed and height where it is not constrained.
Which is cheaper to build?+
A flat roof is usually cheaper in materials and structure for a given footprint, but the gap narrows once a good-quality membrane and adequate insulation thickness are specified rather than a minimal build-up.
Talk through your extension roof
We'll check what sits above the proposed extension, model both roof options against it, and quote in writing for the one that actually works with your windows.