Dormer
A box-shaped extension out the back of the roof, with vertical walls and a window. The default UK loft-conversion shape because it adds the standing-height space needed for a usable bedroom.
A dormer is a box-shaped extension out of a sloping roof, with vertical walls and a window. The dormer's purpose is to add standing-height floor space inside a loft conversion — without it, the sloping roof gives only Velux-lit storage space rather than a usable bedroom.
Common UK dormer types
- Rear dormer — the default. A single box at the back of the roof, often spanning most of the rear face. Most common because it's least visible from the street.
- L-shaped dormer — wraps around the rear and side of a Victorian or Edwardian terrace's roof, taking advantage of the existing rear addition's flat-roof.
- Side dormer — a smaller dormer on the side of the roof, less common because it tends to need full planning permission rather than permitted development.
- Hip-to-gable + rear dormer — raises the side of the roof from a hipped slope to a vertical gable, then adds a rear dormer. Common on 1930s semis, gives the most usable floor area.
Where Offrly fits
Adding a dormer is the most common way to make a loft conversion habitable as an extra bedroom — and the extra bedroom typically commands the 15–20% UK loft-conversion uplift. Model the uplift on your specific home with the Offrly Scenario Explorer: drag the bedroom slider up by one and the floor-area slider up by 20–30 m².
Read the full loft conversion value guide for the 2026 picture.
Indicative market guidance — not a regulated valuation and not financial, tax or legal advice. For mortgage, insurance, probate or tax purposes, consult a RICS-qualified surveyor and an independent qualified adviser. For build cost estimates use a Federation of Master Builders member or a quantity surveyor.
What is a dormer on a UK house?
A dormer is a box-shaped roof extension with vertical walls and a window. It's added to a sloping roof to create standing-height floor space inside a loft conversion. The most common UK loft conversion is a rear dormer, which gives the loft enough headroom to be used as a bedroom.
Does a rear dormer need planning permission?
Rear dormers often fall under permitted development in England, subject to volume limits (40 m³ on terraced houses, 50 m³ on semis and detached), materials matching the existing roof, no work fronting the highway, and side-facing windows being obscure-glazed. Conservation areas, listed buildings and Article 4 directions remove permitted development. Building regulations approval is required regardless of planning status.
What's an L-shaped dormer?
An L-shaped (or hip-to-gable-plus-rear) dormer wraps around the back and side of a Victorian or Edwardian terrace's roof, taking advantage of the existing rear addition. It's the most common loft-conversion shape on London period terraces because it maximises usable floor area on a small footprint.