How to Plan a Rear Dormer Step by Step (2026)
A rear dormer is the most popular loft conversion in the UK because it adds full-height space and is often permitted development. Planning one comes down to six steps: check the head height and feasibility, confirm permissions, design the dormer and windows, get the structural design through building control, compare vetted quotes, then build and sign off. This guide walks through each one.
Planning a rear dormer — the short answer
- Step 1: Check you have at least 2.2–2.4m of ridge head height before anything else.
- Step 2: Confirm permissions — most rear dormers are permitted development, but conservation areas and flats are not.
- Step 3: Design the dormer footprint, roof type (flat or pitched) and window layout.
- Step 4: Get a structural engineer's design and submit a building control application.
- Step 5 & 6: Compare vetted loft-specialist quotes, then build, inspect and collect your completion certificate.
The two things that derail a rear dormer are insufficient head height and assuming planning permission when you actually fall outside permitted development. Confirm both at the very start and the rest of the project runs smoothly.
The 6 Steps to Plan a Rear Dormer
Check head height & feasibility
Measure from the existing ceiling joist to the ridge. You generally need at least 2.2–2.4m at the ridge for a usable rear dormer once the new floor and ceiling are built up. A loft specialist can confirm whether the roof structure suits a dormer.
Confirm permissions
Most rear dormers fall under permitted development if they stay within the volume allowance (40m³ for terraced, 50m³ for semi/detached) and sit below the ridge. Conservation areas, flats and maisonettes need full planning permission, so check with your council first.
Design the dormer & windows
Decide the dormer width, whether the roof is flat or pitched, and the window layout. A full-width flat-roof dormer maximises floor space; matching the window style to the rest of the house keeps it looking right. Plan the staircase position at this stage too.
Structural design & building control
A structural engineer specifies the steel beams and floor joists that carry the new room. Submit a building control application (full plans or building notice) — it covers fire safety, insulation, the staircase and escape windows. This approval is separate from planning.
Compare vetted quotes
Get at least three quotes against the same brief — dormer size, window spec, en-suite or not, insulation and finish — so you compare like-for-like. Check each includes building control fees, scaffolding and making good.
Build, inspect & sign off
A typical rear dormer takes 5–8 weeks. The building inspector visits at key stages; keep the final completion certificate and any structural calculations safe — you will need them when you sell.
Rear Dormer Design at a Glance
Flat-roof dormer
The most common and cost-effective choice. A full-width flat dormer gives the maximum usable floor area and head height for the money.
Pitched-roof dormer
More attractive and often required in conservation areas, but it costs more and gives slightly less internal space than a flat dormer.
Head height first
If the ridge height is marginal, a dormer alone may not give enough standing room. Confirm the measurement before you commit to a design.
Don't forget the stairs
Building Regulations require a fixed staircase with adequate headroom. Where it lands often dictates the whole layout, so plan it early.
Common Questions
Related Guides
More how-to, compare and cost guides for your loft project.
Get 3 Free Rear Dormer Quotes
BestBuilders matches you with 3 vetted loft specialists who price your rear dormer against one brief so you compare like-for-like — dormer size, windows, insulation and building control included. Insurance-backed workmanship.