Dormer Cost Calculator
Calculate the estimated cost of your dormer loft conversion project. This calculator provides a detailed breakdown based on current UK market rates for 2025.
Additional Features
Project Extras
How to Use This Calculator
Select Dormer Type
Choose from flat roof, pitched roof, shed, gable, or hipped dormers. Each type has different structural requirements and costs. Flat roof dormers are typically the most affordable, whilst hipped dormers require more complex construction.
Enter Floor Area
Measure the usable floor space you plan to create in square metres. Typical dormer conversions range from 15m² to 40m². Larger areas require more materials and labour, increasing the overall cost proportionally.
Choose Quality Level
Standard finish includes basic fittings and materials. Premium adds higher quality fixtures and finishes. Luxury includes bespoke joinery, designer fixtures, and top-tier materials throughout.
Add Features
Select additional features like en-suite bathrooms, underfloor heating, or Velux windows. Each feature adds functionality and value but increases the total project cost. Consider which features are essential for your needs.
Dormer Types Explained
| Dormer Type | Description | Cost per m² | Best For |
|---|---|---|---|
| Flat Roof Dormer | Simple box structure with a flat roof, most common in UK conversions | £1,200 – £1,500 | Maximising headroom and floor space economically |
| Pitched Roof Dormer | Features a sloped roof matching the main house style | £1,400 – £1,700 | Properties where aesthetics and planning permission are concerns |
| Shed Dormer | Single sloping roof extending from the main roof ridge | £1,100 – £1,400 | Creating maximum space along one side of the roof |
| Gable Dormer | Triangular front with a pitched roof, traditional appearance | £1,500 – £1,800 | Period properties requiring character preservation |
| Hipped Dormer | Roof slopes back on three sides for a softer look | £1,600 – £1,900 | Semi-detached homes and conservation areas |
Key Cost Factors
Location
London and South East England typically cost 15-30% more than other UK regions due to higher labour rates and material transport costs.
Property Type
Terraced houses often require party wall agreements (£700-£1,500). Detached properties may have easier access but larger roof areas.
Roof Structure
Older properties with traditional timber frames may need reinforcement. Modern truss roofs typically require more extensive structural work (£3,000-£8,000).
Access
Poor site access increases scaffolding complexity and labour time. Narrow streets or restricted parking can add 10-15% to costs.
Materials
Standard materials are budget-friendly. Premium options like natural slate roofing or hardwood windows can double material costs.
Utilities
Extending plumbing, heating, and electrics into the loft space. Bathroom additions require £3,000-£8,000 depending on fixtures and fittings.
Project Timeline
A typical dormer loft conversion follows this schedule:
- Week 1: Scaffolding erection and site preparation. Existing roof covering removal in the conversion area.
- Weeks 2-3: Structural work including steelwork installation, roof alterations, and dormer frame construction.
- Week 4: Roofing and weatherproofing. Installation of windows and external cladding.
- Weeks 5-6: First fix electrics, plumbing, and heating. Insulation and plasterboard installation.
- Weeks 7-8: Plastering, staircase fitting, and second fix services (light fittings, sockets, radiators).
- Weeks 9-10: Decorating, flooring, final fixtures, and snagging. Scaffolding removal and completion.
Most dormer conversions take 8-12 weeks depending on size and complexity. Weather conditions, material delivery delays, and scope changes can extend this timeline.
Planning Permission & Regulations
Many dormer conversions fall under Permitted Development rights, but there are restrictions:
Permitted Development Criteria
- No more than 40 cubic metres additional volume for terraced houses (50 cubic metres for detached/semi-detached)
- No extension beyond the plane of the existing roof slope facing the highway
- Materials must be similar in appearance to the existing house
- No verandas, balconies, or raised platforms
- Side-facing windows must be obscure-glazed and non-opening unless 1.7m above the floor
When Planning Permission Is Required
- Properties in conservation areas, Areas of Outstanding Natural Beauty (AONB), or National Parks
- Listed buildings (also requires listed building consent)
- Flats, maisonettes, or buildings converted into flats
- Dormers that exceed Permitted Development limits
All loft conversions require Building Regulations approval covering structural integrity, fire safety, insulation, ventilation, and staircase design. Expect to pay £800-£1,500 for Building Control fees.
Frequently Asked Questions
How much does a dormer loft conversion cost in the UK?
In 2025, expect to pay between £30,000 and £60,000 for a complete dormer loft conversion. The average cost is approximately £45,000 for a standard 25-30m² conversion with a double bedroom. Premium specifications with en-suite facilities can reach £60,000-£80,000. Costs vary significantly based on location, with London projects typically 20-30% more expensive than the national average.
What’s the cheapest type of dormer?
Flat roof dormers are typically the most affordable option, costing £1,200-£1,500 per m². They have a simple box construction requiring less complex carpentry and fewer materials than pitched alternatives. Shed dormers are similarly economical at £1,100-£1,400 per m². However, consider that some conservation areas may not permit flat roof dormers due to aesthetic guidelines.
Do I need planning permission for a dormer?
Most rear dormers fall under Permitted Development rights and don’t require planning permission, provided they meet specific criteria including size limits, setback from eaves, and material matching. Front-facing dormers, properties in conservation areas, and listed buildings typically require full planning permission. Always check with your local planning authority before starting work, as violating planning rules can result in enforcement action requiring removal.
How long does a dormer conversion take?
A typical single dormer conversion takes 8-10 weeks from start to completion. This includes scaffolding, structural work, roofing, first and second fix services, and finishing. Larger projects or those with complex features like en-suites may take 10-12 weeks. Weather delays, material shortages, or design changes can extend this timeline. Multiple dormers or combined hip-to-gable conversions may require 12-14 weeks.
Will a dormer add value to my home?
A well-executed dormer loft conversion typically adds 10-20% to property value, often exceeding the project cost. In areas with high property values, the return on investment is usually stronger. The conversion must be finished to a good standard with proper access, lighting, and ceiling height. Adding an en-suite bathroom or creating a master bedroom suite typically provides the best value return.
What’s included in the cost?
A comprehensive dormer conversion price includes structural alterations, dormer construction, roofing, windows, insulation, plasterboard, plastering, electrics, heating, flooring, decorating, and staircase. Additional costs often arise from scaffolding (£1,500-£3,000), Building Regulations approval (£800-£1,500), structural engineer fees (£500-£1,500), and any required planning permission (£206 for householder applications). VAT at 20% applies to all work on habitable residential properties.
Can I convert a loft with a truss roof?
Yes, but truss roof conversions are more complex and expensive than traditional cut roofs. Trusses provide structural support across the entire roof, so converting the space requires installing new steelwork to redistribute the load. This adds £3,000-£8,000 to the project cost. A structural engineer must design the steel framework, and Building Control approval is essential. Despite the extra cost, truss roof conversions are entirely feasible and commonly undertaken.
What’s the minimum ceiling height required?
Building Regulations require a minimum ceiling height of 2.2 metres over at least 50% of the floor area. For comfortable living space, aim for 2.3-2.4 metres at the ridge. Measure from your existing joists to the roof ridge to determine feasibility. If insufficient height exists, you can lower the ceiling of the room below (maximum 300mm without structural issues) or raise the ridge height, though the latter often requires planning permission.
Cost Comparison by Size
| Size | Typical Use | Standard Finish | Premium Finish | Duration |
|---|---|---|---|---|
| 15-20 m² | Single bedroom or home office | £24,000 – £32,000 | £32,000 – £42,000 | 6-8 weeks |
| 20-25 m² | Double bedroom | £32,000 – £42,000 | £42,000 – £52,000 | 8-10 weeks |
| 25-30 m² | Double bedroom with en-suite | £40,000 – £52,000 | £52,000 – £65,000 | 9-11 weeks |
| 30-40 m² | Master suite with large en-suite | £50,000 – £68,000 | £68,000 – £88,000 | 10-12 weeks |
| 40+ m² | Multiple rooms or open-plan space | £65,000 – £90,000 | £90,000 – £120,000+ | 12-14 weeks |
Prices include all construction work, finishes, and standard fixtures. Additional features like high-end bathrooms, bespoke joinery, or smart home integration will increase costs further.
Money-Saving Strategies
- Schedule off-peak: Construction firms may offer discounts during quieter winter months (excluding December). Summer is peak season with highest prices.
- Use standard dimensions: Custom window sizes and non-standard materials increase costs. Stick to readily available sizes and finishes where possible.
- Minimise bathroom complexity: Position plumbing close to existing pipes. Simple shower rooms cost £3,000-£5,000 versus £6,000-£10,000 for full bathrooms with separate bath and shower.
- Handle decorating yourself: Professional decorating costs £1,500-£3,000. DIY painting and finishing can reduce the total bill by 5-7%.
- Choose flat roof dormers: Save £3,000-£8,000 versus pitched or hipped alternatives if planning restrictions allow.
- Retain existing staircase location: Moving staircase positions requires structural alterations to the floor below, adding £2,000-£5,000.
- Obtain multiple quotes: Price variation between contractors can be 20-30%. Get at least three detailed quotes and check references thoroughly.
- Avoid scope creep: Design changes mid-project increase costs significantly. Finalise all decisions before work commences.
Quality Checklist
Verify your contractor addresses these critical areas:
Structural Integrity
- Structural engineer’s calculations approved by Building Control
- Steel beams certified and installed to specification
- Existing floor joists reinforced if necessary (usually required for truss roofs)
- Load-bearing walls properly supported throughout construction
Weatherproofing
- High-quality breathable membrane under roof covering
- Lead flashing correctly fitted around dormer joins
- Windows and doors with appropriate weather seals
- Guttering and downpipes adequate for increased roof area
Insulation & Ventilation
- Minimum 270mm insulation between and under rafters (current Building Regulations)
- Continuous insulation with no cold bridges
- Adequate ventilation to prevent condensation (trickle vents in windows minimum)
- Vapour control layer correctly positioned
Fire Safety
- Protected escape route from loft space (30-minute fire-resistant construction)
- Fire doors fitted to all habitable rooms opening onto the staircase
- Smoke alarms on each floor interlinked
- Windows suitable for emergency escape or alternative escape route provided
Services
- Electrical work certified by qualified electrician (Part P compliance)
- Adequate heating provision for the new space
- Plumbing tested and certified leak-free
- All services accessible for future maintenance
References
Royal Institution of Chartered Surveyors (RICS) – Building Cost Information Service (BCIS), 2025 Construction Price Data
HM Government Planning Portal – Permitted Development Rights for Householder Applications, Updated 2024
Building Regulations Approved Documents: Part A (Structure), Part B (Fire Safety), Part C (Site Preparation and Resistance to Contaminants and Moisture), Part F (Ventilation), Part L (Conservation of Fuel and Power)
Federation of Master Builders (FMB) – Loft Conversion Cost Guide, 2025 Edition