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Dormer loft conversion estimate.

Dormer loft conversions are the most popular way to add headroom and usable floor space. This guide breaks down the costs of flat-roof and pitched-roof dormers, including structural trimming, cladding, and fit-out.

Authored by Alexandr Vreme & Maksym Vasylkov
Published July 2026 — Next review July 2027

Who this guide is for

For builders and estimators pricing dormer loft conversions specifically. Whether flat-roof or pitched-roof, full-width or part-width, dormers involve structural trimming of the existing roof that differs from other conversion types.

Dormer types

• Flat-roof dormer — most common and most economical. A flat roof with a slight fall, clad in GRP, EPDM, or felt.
• Pitched-roof dormer — a small pitched roof matching the main roof profile. More expensive but blends in better aesthetically.
• Full-width dormer — spans the entire rear elevation, maximising floor area.
• Part-width dormer — narrower than the full elevation, leaving roof slope visible on either side.

Information you need before pricing

• Existing roof pitch and covering material — understanding the junction detail
• Dormer width and projection — how far does it extend from the roof slope?
• Cladding material — render, tile hanging, timber, or composite panels
• Window specification — standard uPVC, aluminium, or timber casements
• Flat roof covering — GRP (glass-reinforced plastic), EPDM rubber, or built-up felt

Structural works

• RSJ trimming — steel beams cut into the existing roof structure to create the dormer opening
• Dormer walls — timber-frame or blockwork walls built off the trimmed opening
• Roof covering — flat roof system or mini-pitched roof over the dormer projection
• Insulation — meeting building regs U-values in walls, roof, and ceiling

Typical cost ranges by element

• Structural trimming (steel, cutting rafters, padstones): £3,000–£6,000
• Dormer carcass (timber frame, walls, sheathing): £4,000–£8,000
• Windows and external doors: £2,000–£5,000
• Cladding (render, tile hang, or timber): £3,000–£6,000
• Roof covering (GRP, EPDM, or felt system): £2,000–£4,000
• Internal fit-out (insulation, plaster, first and second fix, decoration): £15,000–£25,000

Commonly missed costs

• Scaffold access for dormer construction — often needs a larger scaffold than a simple roof window installation
• Additional steel to support the dormer projection beyond the roof line
• Upgrading existing roof structure — rafters may need reinforcement to carry dormer loads
• Making good roof tiles around the dormer junction — lead flashings and code-compliant detailing
• Temporary roof cover if the dormer opening is cut before the shell is weathertight

Miniature worked example

A 3.5 m wide flat-roof dormer on a semi-detached house (dormer cost only):

• Structural trimming (RSJ, padstones, cutting rafters): £5,000
• Dormer shell (timber frame, walls, sheathing): £10,000
• Roof and cladding (GRP roof, tile hanging): £6,000
• Windows (aluminium casements): £3,000
• Internal fit-out (insulation, plaster, electrics, decoration): £18,000

Dormer element: approximately £42,000. The total conversion cost, including staircase installation, landing alterations, and making good below, would be in the region of £55,000. Pitched-roof dormers add £4,000–£8,000 due to more complex roof joins.

Suitable output documents

• Detailed estimate with dormer element separated from the rest of the conversion
• Priced bill of quantities itemising steelwork, cladding, and roof covering
• Schedule of works with structural hold points before weathertight stage

Questions to ask before pricing

• Flat-roof or pitched-roof dormer? The cost difference is significant.
• Full-width or part-width dormer?
• What cladding finish does the client want — render, tile hanging, or timber?
• Has a structural engineer surveyed the existing roof to confirm trimming feasibility?