Loft Conversion Cost Calculator

Conversion Type

Width

Depth

Region

Property Type

Roof Type

Additional Dormers

En-suite+£6,500

Estimated Dormer Conversion Cost (incl. VAT)

£103,680

£2,160/m²159% above UK average

Floor area: 48.0 m² — South East

Cost Breakdown

How your estimate is distributed across work categories

Structural Work

£36,288(35%)

Windows & Rooflights

£15,552(15%)

Insulation & Plastering

£12,442(12%)

Staircase

£10,368(10%)

Electrics & Plumbing

£10,368(10%)

Decoration & Finishing

£8,294(8%)

Fees & Permissions

£5,184(5%)

Contingency

£5,184(5%)

Project Overview

Timeline, property value impact, and planning requirements

6–8

Weeks

Dormer conversion

+£49,875

Value Uplift

~18% of UK average (£285K)

Planning PermissionCheck with Local Authority

Dormer and hip-to-gable conversions may require planning permission depending on your property type, location (conservation areas), and existing extensions. Check with your local planning authority.

Compare Conversion Types

See how other conversion types compare for the same floor area

Velux / RooflightSave £34,560

How Much Does a Loft Conversion Cost in 2026?

UK loft conversion costs by type, with regional variations and typical ranges.

A loft conversion is one of the most cost-effective ways to add living space to your home. In 2026, UK loft conversion costs range from £20,000 for a basic Velux conversion to £70,000+ for a full Mansard conversion in London. The average across the UK is approximately £40,000.

£20K–35K

Velux / Rooflight

Simplest option

£35K–55K

Dormer

How to Use This Loft Conversion Cost Calculator

Three simple steps to get your personalised estimate.

Pick your conversion type. Velux (rooflights only, no structural changes to the roof) is the most affordable. Dormer extends outward to create headroom. Hip-to-Gable extends sideways. Mansard replaces one whole roof slope — the most expensive but maximises space.
Enter your property dimensions and region. The calculator uses your floor area and regional cost multipliers. London and the South East are the most expensive; Wales and Northern Ireland are the most affordable.
Toggle optional extras. Add an en-suite bathroom, planning permission fees, party wall agreements, or a premium finish. Toggle VAT on/off to see the net and gross costs.

How the estimate is calculated:

Total = (Floor Area × Base Rate × Region × Property × Roof) + Dormers + En-suite + Fees + VAT

Loft Conversion Cost by Type

Each conversion type has different costs, timelines, and space benefits.

Velux / Rooflight£1,200/m² | 4–6 weeks

Rooflights are installed into the existing roof slope with minimal structural work. No dormers, no roof alteration. Ideal for lofts with good existing headroom. Falls under permitted development in most cases.

Dormer£1,800/m² | 6–8 weeks

A box-shaped extension that projects from the roof slope, creating full headroom and usable floor space. Flat-roof dormers are most common. Can add multiple dormers. The most popular UK loft conversion type.

Hip-to-Gable£2,100/m² | 7–10 weeks

Extends the sloping (hipped) side of the roof outward to create a vertical gable wall. Only applicable to semi-detached or detached homes with hipped roofs. Adds significant internal volume.

Mansard£2,500/m² | 8–12 weeks

Replaces one entire roof slope with a near-vertical wall and a shallow-sloped roof. Creates the maximum possible living space. Almost always requires planning permission. Common in London conservation areas.

Factors That Affect Loft Conversion Costs

Location, property type, roof structure, and finish level all impact the final price.

Region

London costs 35% more than the South East baseline. The North of England, Scotland, Wales, and Northern Ireland are 15–30% cheaper.

Property Type

Detached homes cost ~5% more due to access on all sides. Terraced homes cost ~8% less. Bungalows cost ~10% more for structural reinforcement.

Roof Structure

Hipped roofs cost ~8% more than gable roofs due to additional structural work. Trussed rafters require more steelwork than traditional cut timber roofs.

Finish Level

A premium finish (high-end bathroom, bespoke joinery, underfloor heating) adds approximately 15% to the base cost.

Planning Permission & Building Regulations

What you need to know before starting your loft conversion.

Permitted Development: Most Velux/rooflight conversions and many dormer conversions fall under permitted development, meaning you do not need planning permission — provided you stay within volume limits (40m³ for terraced, 50m³ for detached/semi-detached), don't extend beyond the existing roof plane at the front, and use matching materials.

When Planning Permission IS Required: Mansard conversions almost always need it. You'll also need it if you live in a conservation area, a listed building, or if your plans exceed permitted development limits. Budget £1,500–£3,000 and allow 8–12 weeks.

All loft conversions require Building Regulations approval — this is separate from planning permission and covers structural safety, fire escape, insulation, and stairs. Budget £750–£1,500 for building control fees.

Common Mistakes to Avoid

Pitfalls that can blow your budget or delay your project.

Underestimating structural work

Steel beams (RSJs) are needed to support the new floor and roof. They typically cost £1,500–£3,000 including installation. Skipping this in your budget can add 20–30% to costs mid-project.

Ignoring the party wall agreement

If your property shares a wall with a neighbour (semi-detached/terraced), you legally need a Party Wall Agreement for structural work affecting that wall. Budget £1,200–£2,000 and allow 2–3 months for the process.

Forgetting about the staircase

Building regulations require a proper staircase (not a ladder or space-saver) for loft conversions used as habitable rooms. A staircase costs £3,000–£8,000 and takes up floor space on the floor below — plan for this.

Not budgeting for decoration

Structural work is just the start. Plastering, painting, flooring, electrics, and plumbing for an en-suite can add £8,000–£15,000 to the final bill.

Skipping the contingency

Always reserve 10–15% of your budget for unforeseen issues: rotten timbers, asbestos, rewiring, or upgrading the existing ceiling structure. Loft conversions in older homes frequently uncover surprises.

Frequently Asked Questions

Common questions and detailed answers

UK loft conversion costs in 2026 range from £20,000 to £80,000 depending on the type. A basic Velux/rooflight conversion costs £20,000–£35,000. A dormer conversion (the most popular type) costs £35,000–£55,000. A hip-to-gable conversion costs £45,000–£65,000. A full mansard conversion costs £55,000–£80,000. Costs vary significantly by region — London is typically 35% more expensive than the South East baseline. Use the calculator above to get a personalised estimate based on your exact property dimensions and location.

A Velux (rooflight) conversion is the cheapest at £20,000–£35,000. It requires no structural changes to the roof — rooflights are simply installed into the existing roof slope, and the interior is fitted out. This works best for lofts that already have good headroom (at least 2.2m at the highest point). The trade-off is that you get less usable floor space compared to a dormer or mansard conversion.

Most loft conversions fall under permitted development rights, meaning planning permission is not required — as long as you stay within these limits: the additional volume does not exceed 40m³ for terraced houses or 50m³ for detached/semi-detached houses; the extension does not extend beyond the existing roof plane on the principal elevation; materials match the existing house; and no part is higher than the highest part of the existing roof. However, if you live in a conservation area, a listed building, or are planning a mansard conversion, you will almost certainly need planning permission. Always check with your local planning authority before starting work.

Project timelines vary by conversion type. A Velux/rooflight conversion takes 4–6 weeks. A dormer conversion takes 6–8 weeks. A hip-to-gable conversion takes 7–10 weeks. A mansard conversion takes 8–12 weeks. Adding an en-suite bathroom typically adds 1–2 weeks. Most of the structural work (steels, dormer construction, roof) is done in the first 2–3 weeks, with the remaining time spent on insulation, plastering, electrics, plumbing, stairs, and finishing.

A loft conversion typically adds 15–20% to a property's value, according to Nationwide and RICS research. On the 2026 UK average house price of £285,000, that's an uplift of £42,750–£57,000 — often more than the cost of the conversion itself. The exact uplift depends on the quality of the conversion, the number of bedrooms added, whether an en-suite is included, and the local property market. A well-executed loft conversion with an additional bedroom and bathroom in a family-friendly area typically delivers the best return on investment.

A Velux (rooflight) conversion installs windows into the existing roof slope without altering the roof structure. It's the simplest, cheapest, and fastest option, and usually falls under permitted development. A dormer conversion extends the roof outward to create a box-shaped projection with a vertical window and a flat or pitched roof. Dormers create more usable floor space and full headroom but cost more (£35,000–£55,000 vs £20,000–£35,000 for Velux) and may require planning permission. Dormers are the most popular loft conversion type in the UK because they offer the best balance of cost, space, and light.

A dormer loft conversion in the UK costs £35,000–£55,000 on average (2026 prices, including VAT). The cost per square metre is approximately £1,800. The final price depends on the number of dormers (each additional dormer adds ~£4,000), your region (London is 35% more expensive), your property type, and the finish level. A typical 2-bedroom dormer conversion with an en-suite bathroom on a semi-detached house in the South East costs around £45,000–£50,000.

Yes, terraced houses are very commonly converted. In fact, terraced house loft conversions can be slightly cheaper (about 8% less) due to the shared party walls reducing some structural costs. The key considerations for terraced houses are: you will almost certainly need a Party Wall Agreement with your neighbours (budget £1,200–£2,000); access for materials may be more restricted (through the house rather than externally); and the permitted development volume limit is 40m³ rather than 50m³ for detached/semi-detached homes. Many Victorian and Edwardian terraces in London and other UK cities have been successfully converted.

A Party Wall Agreement is a legal document required under the Party Wall etc. Act 1996 when building work affects a wall shared with a neighbour (common in semi-detached and terraced houses). For loft conversions, you'll need one if: you're cutting into a party wall to insert steel beams, raising the party wall, or excavating near a neighbour's foundation. The agreement requires a Party Wall Surveyor (typically £1,200–£2,000 total) and must be served at least 2 months before work begins. We strongly recommend discussing plans with neighbours informally before serving formal notice — it prevents disputes and delays.

Yes, most loft conversions are standard-rated for VAT at 20%. However, there are some exceptions: if the property has been empty for 2+ years, a reduced rate of 5% may apply; if you're converting a non-residential building into a home, the zero rate may apply; and some energy-saving materials (insulation, solar panels) qualify for a reduced 5% rate. Most homeowners will pay the full 20% VAT, which is already included in our calculator by default. You can toggle VAT off in the 'Options & Fees' section to see the net cost.

A mansard loft conversion costs £55,000–£80,000 in the UK (2026 prices). The cost per square metre is approximately £2,500 — the highest of all conversion types. Mansard conversions replace one entire roof slope with a near-vertical wall and a shallow-pitched roof, creating maximum living space. They are most common in London (particularly in conservation areas like Kensington, Chelsea, and Westminster) where maximising space on expensive land justifies the higher cost. They almost always require planning permission and take 8–12 weeks to complete.

The biggest factors affecting loft conversion costs are: (1) Conversion type — Velux is cheapest, Mansard is most expensive, with a 2× cost difference between them. (2) Region — costs in London are 35% above the national baseline, while costs in Wales and Northern Ireland are 25–30% below. (3) Floor area — a larger loft costs more overall but less per square metre due to fixed costs being spread. (4) Structural work — steel beams (RSJs), floor reinforcement, and roof alterations can add £5,000–£15,000. (5) Finish level — a premium finish with bespoke joinery, underfloor heating, and a high-end en-suite can add 15–25%. (6) Add-ons — each additional dormer, en-suite bathroom, or planning requirement adds fixed costs.

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Last updated May 26, 2026