What Does a Dormer Cost?

Cost by Dormer Type

• Small single dormer (window only): £5,000–12,000
• Side dormer: £20,000–35,000
• Rear flat-roof dormer (full width): £25,000–45,000
• Hip-to-gable + rear dormer: £35,000–55,000
• L-shaped dormer: £40,000–65,000
• Mansard dormer: £45,000–70,000

What’s Included

• Structural steelwork and timber framing
• Roof covering (GRP flat roof, tiles, or lead)
• Windows (typically double-glazed Velux or casement)
• Insulation to Part L standards
• Internal plastering, electrics, and decoration
• Fire safety upgrades (fire doors, smoke alarms)
• Scaffolding (typically £800–1,500)

Not Included

• Planning permission (£258 householder application)
• Party wall surveyor (if applicable): £700–1,500 per neighbour
• Building Regulations application: £200–500
• Architect/designer fees: £1,500–4,000
• Bathroom/kitchen fit-out if adding en-suite

Planning Permission

Many rear dormers fall under permitted development (PD) rights, but there are conditions: the dormer must not extend above the existing ridge, must be set back from the eaves, and cannot face a highway. Side dormers and front dormers typically require planning permission. Always check with your local planning authority.

Timescale

A typical dormer loft conversion takes 6–10 weeks on site. Allow 4–8 weeks for planning and Building Regulations approval before work starts.