Free Loft Conversion
Cost Calculator 2026
Our free loft conversion cost calculator gives you instant 2026 estimates for every conversion type — dormer, mansard, Velux, L-shaped dormer and hip-to-gable. Furthermore, rates are London-adjusted and updated quarterly. Switch to Total Project Cost to see the real all-in budget including VAT, professional fees and contingency. No sign-up required.
Loft Conversion Types — Costs & Key Facts 2026
Each conversion type involves a different level of structural work and therefore delivers a different amount of usable space. The right choice depends on your roof profile, planning context and budget. Furthermore, the type you choose affects whether you need full planning permission or can proceed under permitted development.
Velux / Rooflight
The most economical option. The existing roof profile is left completely unaltered — only roof windows are inserted. Furthermore, it works best where the loft already has good ridge height (at least 2.3m). As a result, no exterior change means conservation areas rarely cause an issue.
Usually Permitted DevelopmentRear Dormer
The most popular choice for London terraced houses. A box-shaped flat-roofed projection creates full headroom across the rear of the loft. Furthermore, it adds significant usable floor area and consequently accommodates a bathroom or en-suite. It usually falls within permitted development rights.
Usually Permitted DevelopmentL-Shaped Dormer
Combines a rear dormer with a return over the rear outrigger — common on Victorian and Edwardian terraces. It therefore creates the largest possible footprint within permitted development, often delivering two bedrooms with an en-suite in one project.
Usually Permitted DevelopmentHip to Gable
Ideal for 1930s semi-detached and detached homes where the side roof slopes inward. The sloping hip is replaced with a vertical gable wall, which consequently increases usable volume dramatically. It is usually combined with a rear dormer for maximum space.
Usually Permitted DevelopmentMansard
The most extensive conversion type. The entire roof is replaced with near-vertical 70-degree walls and a flat roof, creating a full new storey. However, it always requires full planning permission. It is therefore best suited to period properties in prime London locations where the value uplift justifies the cost.
Planning Permission RequiredTotal Project Cost
The real all-in budget typically runs 40% above the builder's quote. Furthermore, you need to add 20% VAT, 8–12% architect and structural engineer fees, building control fees and a 10% contingency. As a result, always use our Total Project Cost toggle above for a complete budget picture.
Always Budget for ThisWhat Is Included in This Estimate?
The build cost estimate covers the main construction work. However, several important costs sit outside the estimate and must be budgeted separately. Consequently, failing to include them is the most common reason homeowners run over budget on loft conversion projects.
✅ Included in Build Estimate
- ✓Structural alterations to existing roof and joists
- ✓New dormer structure, cladding and flat roof
- ✓Roof windows, skylights and dormer glazing
- ✓Insulation, airtightness membrane and boarding
- ✓New loft staircase and landing
- ✓Plasterboard, plastering and decoration
- ✓First and second fix carpentry
- ✓Electrics and lighting to new loft space
- ✓Bathroom suite where selected above
❌ Excluded — Budget Separately
- ✗VAT at 20% on all construction works
- ✗Architect fees — drawings, planning, building control
- ✗Structural engineer fees (£800–£2,000)
- ✗Planning application fee (£548 for householder applications)
- ✗Building control / building regulations fees
- ✗Party wall surveyor fees where required
- ✗Flooring, fitted wardrobes and bespoke joinery
- ✗10% contingency buffer (always add this)
Loft Conversion Cost Calculator FAQs
About This Loft Conversion Cost Calculator — London 2026
Signature Build Properties maintains this loft conversion cost calculator as a free tool for homeowners planning loft projects across West and South West London. Our team has completed loft conversions across Ealing, Hounslow, Chiswick, Richmond, Hammersmith, Fulham and Putney for over 15 years. As a result, the rates in this calculator come from real tender returns and live project costs — not desk-based national surveys.
How We Set Our Rates
We update base rates quarterly against BCIS tender price data and our own project results. Furthermore, the calculator covers all five principal loft conversion types: Velux rooflight conversions (from £1,400/m²), rear dormer conversions (from £2,400/m²), L-shaped dormers (from £2,700/m²), hip-to-gable conversions (from £2,550/m²) and mansard conversions (from £3,100/m²). In addition, all projects are assessed against UK planning regulations to confirm permitted development eligibility before work begins.
Build Cost vs Total Project Cost
In the default view, results show the build cost only and consequently exclude VAT and professional fees. However, the true all-in cost runs significantly higher. Therefore, switch to Total Project Cost to see the real budget including 20% VAT, 10% professional fees and 10% contingency. This is the figure you should take to your bank or financial advisor when planning your project — not the builder's quote alone.
Book a Free Loft Survey
Results give you an accuracy of ±15–20% and work well as a feasibility guide. However, for a precise fixed-price figure, you need a site visit and a written specification. Therefore, call +44 7404 931629 or use the contact form to book a free survey. Moreover, all our loft conversions use fixed-price contracts — so you know the total cost before work starts on site.
← View All Construction CalculatorsFree Loft Survey —
West London This Week
Our loft conversion team will visit your property and assess the feasibility. Furthermore, we confirm your permitted development position and consequently provide a detailed fixed-price quote. No obligation.
