Flat-roof dormer Cost Calculator

How this calculator works

Every figure in this calculator is the one published in the cost table on our flat-roof dormer guide. Pick the option that matches your job and the calculator returns that published range, adjusted for your region - it does not model anything the guide does not already say.

That is deliberate. A calculator whose rates drift from the page it sits under is worse than no calculator, because two numbers on the same site disagree.

What it cannot know: access, the condition of what is already there, and anything structural. Those are what separate three real quotes from each other.

What you’re actually paying a builder to construct

A flat-roof dormer is a box that’s framed out from the existing slope. Timber studwork forms the cheeks – the side walls – and the front, then a flat deck goes on top. That deck is usually covered in EPDM rubber, GRP fibreglass or built-up felt; rubber and fibreglass last longer and are what most builders fit now.

The point of the box is head height. A pitched roof steals space where it slopes down to the eaves, so pushing a vertical wall out reclaims it – you end up with proper standing room and usable floor across more of the plan. That’s where the extra habitable square metres behind the figures in the cost table come from.

Width decides whether steel is involved. A modest single dormer often sits on timber alone. Once you go full-width, or twin dormers across the front, the opening in the existing roof gets wide enough that a steel beam has to carry the ridge and the loads above it – and steel means a structural engineer’s calculations and a bigger day rate.

Matching the rest of the house is quiet money well spent. The cheeks can be tile-hung or clad to echo the existing roof, and the window sizes and reveal depth change how the dormer reads from the street. Cheap frames and a mismatched finish are the usual tell that a dormer was built down to a price.

Many rear dormers fall under permitted development, so you may not need a full planning application – but volume limits apply, and anything facing a road, or any flat or maisonette, usually does need consent. Check what your roof qualifies for on the Planning Portal loft conversion guidance before you assume. If a cheek sits on or near the boundary, you’ll also have a party wall notice to serve on the neighbour.