Rear and side dormer loft conversions across Barnet and Hertfordshire — the workhorse conversion for 1930s semis and Edwardian terraces. Adds a full-height double bedroom and en-suite without changing the existing house footprint.
What a Dormer Conversion Is
A dormer extension projects out from the existing roof slope, typically at the rear of the house. The new walls go up from the ceiling joists of the first floor, creating a flat-ceilinged room with full standing height across most of its footprint. It's the most popular conversion type in Barnet for one reason: it gives you a proper double bedroom, not a low-ceilinged room with windows in the slope.
When a Dormer Works Best
1930s and 1950s semis. The classic dormer house. Pitched roof at the back, terraced ceiling line, plenty of head height once the dormer goes in. Borehamwood, Mill Hill, East Barnet, large parts of Finchley.
Edwardian and Victorian terraces. Yes, but planning constraints are tighter and party-wall agreements with both neighbours are mandatory. High Barnet, Whetstone, parts of Finchley N3.
Bungalows. A dormer can effectively double the floor area of a bungalow. Very popular in pockets of New Barnet and Hadley Wood.
Permitted Development vs. Planning
Rear dormers are typically permitted development if the volume added doesn't exceed 40m³ for terraced houses or 50m³ for detached and semi-detached. Conservation areas (parts of EN5 around High Barnet, parts of N20) and Article 4 directions remove PD rights — you'll need a planning application. The specialist will check during the survey.
Typical Layout
- One double bedroom (3.5m × 4.5m typical)
- En-suite shower room (~2m × 1.8m)
- New staircase from first-floor landing
- 2–3 Velux roof windows on the original slope, plus 2–3 windows in the dormer face
Cost & Timeline
Rear dormer conversions in Barnet typically cost £30,000–£50,000 all-in, including:
- Structural work (RSJs, new joists)
- Dormer frame, roof, tiling/membrane
- Insulation to current Part L
- Electrics, plumbing, heating
- Plaster, decoration, flooring
- Building Control sign-off
Typical timeline: 8–10 weeks on site, plus 4–6 weeks for design, structural calcs and Building Control submission upfront.
Dormers on Barnet semis
The 1930s semi-detached houses that dominate large parts of Borehamwood, Mill Hill and East Barnet are almost ideal dormer candidates. Generous rear roof slope, standard ridge height, no awkward planning constraints in most postcodes. A standard rear dormer here delivers one double bedroom and an en-suite, with the staircase usually running over the existing landing.
Frequently Asked Questions
Do I need planning permission for a rear dormer?
Usually no — most rear dormers in Barnet fall under permitted development. Conservation areas and Article 4 directions remove PD rights. The specialist checks this on the initial survey.
How disruptive is the build?
The roof comes off for 3–5 days during the structural phase, weather-permitting. The rest of the build is largely contained to the loft floor. Most families stay in the house throughout.
Do I need a party wall agreement?
If your house is semi-detached or terraced, yes — a Party Wall Notice is required for the neighbouring wall and (for terraces) for both neighbours. The specialist will manage this or recommend a party-wall surveyor.
Can a dormer fit a bathroom?
Yes — most rear dormers in Barnet include an en-suite shower room. Adds £4,000–£7,000 to the base cost.