What Size Dormer Do I Need?
Quick Answer
Min. 2.3m head height • Width typically 1.8–3.6m
Building Regulations minimum ceiling height 2.3m over at least 50% of floor area. Check planning requirements
Dormer Types & Typical Sizes
- Single dormer window: 1.2–1.8m wide, adds light and ventilation
- Double dormer: 2.4–3.6m wide, adds usable floor area
- Full-width flat-roof dormer: Spans the full rear roof, maximises space
- Hip-to-gable + dormer: Converts hipped roof to vertical gable, then adds rear dormer
Building Regulations Requirements
- Minimum ceiling height: 2.3m over at least 50% of the habitable floor area
- Stairs: Minimum 2m headroom, maximum 42° pitch for domestic stairs
- Fire escape: Window at least 450 × 450mm opening, sill max 1100mm from floor
- Structural: Steel beams typically required for floor joists and dormer cheeks
Permitted Development Limits
In England, a rear dormer may be permitted development if:
- Volume added does not exceed 40m³ (terrace) or 50m³ (detached/semi)
- Dormer does not extend beyond the existing roof plane facing the highway
- Materials match the existing house
- No balcony or raised platform
- Side-facing windows are obscure-glazed and fixed shut (or top-opening only)
Always verify with your local planning authority — conservation areas, listed buildings, and Article 4 directions remove PD rights.
Last updated: April 2026