Dormer loft conversions

What a dormer actually is

Structurally, a dormer is a new opening cut into the existing roof, framed with timber or steel, then built out as a small box with its own walls, roof and windows. The existing rafters are trimmed and supported, a new floor structure spans between load bearing walls rather than resting on the ceiling joists, and the whole thing is insulated and weathered back into the original roof.

Visually, it changes the back of the house. That is why the type you choose matters as much as the size, and why placing it on the rear slope keeps most projects simple from a planning point of view.

Dormer types compared

Head height: the measurement that decides everything

The number to measure is from the top of the existing ceiling joists to the underside of the ridge. Once the new floor and the insulated ceiling are built, both eat into that dimension, so the finished head height is always less than what you measure in an empty loft.

As a rule of thumb, if the measurement is comfortable there is a straightforward conversion available. If it is marginal, the options are lowering the ceilings in the rooms below, which is disruptive but effective, or raising part of the roof structure, which is a bigger planning and build proposition. Both are decisions to make at survey rather than mid build, and we take that measurement on the first visit.

Building regulations require adequate head height over the stair and a safe route out in a fire, so the stair position often decides the layout before the room does.

Planning permission for a dormer

Rear dormers are frequently permitted development, subject to volume allowances that are smaller for terraced houses than for semis and detached homes, and subject to the dormer not sitting forward of the roof plane facing a highway. Set back from the eaves and materials similar to the existing house are both conditions in practice.

Rights are reduced or removed in conservation areas, on listed buildings, under Article 4 directions and on some newer estates where a planning condition removed them. Much of our patch sits in the High Weald, so this catches more houses here than the national picture suggests.

Building regulations apply either way, covering structure, fire escape, insulation, the staircase and, where relevant, the drainage for an ensuite. Confirm the planning route with your local planning authority or the Planning Portal before you commit to a design.

Read the permitted development guide →

What a dormer costs

A dormer costs more than a rooflight conversion because you are rebuilding part of the roof, and less than a hip to gable because you are not rebuilding a whole end of it. Within dormer projects, the variables that move the price are width, whether an ensuite goes in, the staircase solution and the roof covering you choose.

We survey for free and issue a fixed price written quote that names the steel, the windows, the stair and the building control application individually, so there is nothing left as an allowance to be argued about later.

See the full loft conversion cost guide →

How the build runs

1. Step 1

   Survey and design

   Head height measured, structural approach agreed, stair position resolved on paper before anything else.

2. Step 2

   Approvals

   Planning route confirmed, structural calculations produced and the building regulations application submitted.

3. Step 3

   Scaffold and opening up

   Scaffold with a temporary roof or sheeting so the house stays dry while the roof is opened.

4. Step 4

   Structure

   Steels and new floor in, dormer framed, sheathed and made weathertight. This is the noisy fortnight.

5. Step 5

   Fit out

   Insulation, stair, plasterboard, electrics, any ensuite, then decoration and final building control inspection.