A dormer loft conversion in the UK typically costs between £25,000 and £65,000 in 2026, depending on the type, size, and specification. A small flat roof dormer (around 3m × 3m) starts from around £25,000–£35,000. A medium-sized flat roof dormer (4m × 4m) typically costs £35,000–£50,000. Larger or more complex dormers (gable, hipped, L-shaped, or wrap-around) can cost £45,000–£65,000+. London and South East prices are typically 20–30% higher than the rest of the UK.

The flat roof dormer (also called a box dormer) is the cheapest and most common type of dormer loft conversion in the UK. It typically costs £25,000–£50,000 depending on size and specification. The shed dormer (with a single sloping roof) is the next most affordable at around £22,000–£40,000. More complex designs like gable, hipped, L-shaped, and wrap-around dormers cost more due to additional structural work and materials.

Many dormer loft conversions fall under Permitted Development rights and do not require planning permission, provided they meet specific criteria: the dormer must not extend beyond the existing roof plane at the front of the house, volume additions must not exceed 40 cubic metres (terraced) or 50 cubic metres (semi-detached/detached), materials must be similar in appearance to the existing house, and the property must not be in a conservation area, AONB, or be a listed building. However, building regulations approval is always required. If in doubt, apply for a Lawful Development Certificate from your local council.

A typical dormer loft conversion takes 6–10 weeks from start to finish. A small flat roof dormer can be completed in 4–6 weeks, while larger or more complex dormers (L-shaped, wrap-around) may take 10–14 weeks. This includes structural work, roofing, windows, first and second fix, plastering, and decoration. The timeline can be extended by planning permission requirements (8 weeks), party wall agreements, or adverse weather conditions.

A flat roof dormer has a completely flat (or very slightly pitched for drainage) roof and vertical walls, maximising internal headroom and floor space. It’s the most popular choice for UK loft conversions. A shed dormer has a single sloping roof that slopes in the same direction as the main roof, giving it a more traditional appearance but slightly less internal space. Flat roof dormers are generally cheaper and provide more usable space, while shed dormers may be preferred for aesthetic reasons, especially on period properties.

A dormer loft conversion typically adds 10–20% to the value of your property, depending on the quality of the conversion and your local market. In London and the South East, where space is at a premium, a well-executed dormer conversion adding an extra bedroom and en-suite can increase property values by £50,000–£100,000+. In most cases, the cost of the conversion is recovered within 2–5 years through the increased property value, making it one of the best home improvement investments.

Yes, building regulations approval is always required for a dormer loft conversion, regardless of whether planning permission is needed. Building regulations cover structural integrity, fire safety (including means of escape), insulation (Part L), ventilation, stair design, and electrical safety. You can apply through your local authority building control team or use an approved inspector. Costs for building regulations approval typically range from £300–£800. Your builder or architect will usually handle this process.

A typical dormer loft conversion cost includes: structural steel beams and timber frame, roof construction and covering, dormer windows (usually uPVC double-glazed), insulation (walls, floor, and roof), plasterboarding and plastering, staircase installation, electrical wiring and lighting, plumbing (if adding a bathroom), heating extensions, fire doors and smoke alarms (for building regs compliance), and decoration. Not typically included: planning fees (if required), building control fees, party wall surveyor costs, furniture, and floor coverings.