Loft Conversion Cost UK 2026: dormer, mansard, hip-to-gable and rooflight, priced
What every type of loft conversion should cost in 2026, in London and across the UK, with the spec variables that move the number and the fees most homeowners miss.
Key facts
- Rooflight (Velux): £45,000–£90,000 nationally, £60,000–£110,000 in London.
- Dormer: £55,000–£105,000 nationally, £70,000–£135,000 in London.
- Hip-to-gable + rear dormer: £75,000–£130,000 nationally, £95,000–£165,000 in London.
- Mansard: £85,000–£150,000 nationally, £110,000–£185,000 in London.
- Professional fees add 12–18% (architect, structural engineer, party wall, cost expert, building control).
- VAT at 20% applies to almost all loft conversions on existing homes.
A loft conversion is one of the highest-return improvements you can make to a UK home. It is also one of the easiest projects to overspend on, because the price is driven by structural decisions made before the builder ever picks up a tool. This guide sets out the working 2026 ranges by type, and the variables that move you up or down inside them.
The cheapest loft conversion is the one designed to fit the existing roof, not the one designed to fit your wish list.
Loft conversion cost by type, 2026
The four common types, with national and London ranges. These are construction cost only: contractor's price including labour, materials, prelims and overheads. They exclude VAT and professional fees.
Rooflight (Velux)
£45,000–£90,000 nationally. £60,000–£110,000 in London. The existing roof structure stays. New floor joists, insulation, plasterboard, Velux windows, a staircase, and a bathroom. Only works where the ridge height already gives 2.2m+ of clear space.
Dormer
£55,000–£105,000 nationally. £70,000–£135,000 in London. A rear or side dormer creates standing height and usable floor area where the existing pitch does not. Cost moves with dormer size, cladding choice (zinc, lead, tile-hung) and whether a Juliet balcony is added.
Hip-to-gable plus rear dormer
£75,000–£130,000 nationally. £95,000–£165,000 in London. The hipped end is rebuilt as a vertical gable wall, then a rear dormer is added. Standard on London semi-detached and end-of-terrace homes. The most popular conversion type in Greater London.
Mansard
£85,000–£150,000 nationally. £110,000–£185,000 in London. A near-vertical rear wall is rebuilt to take a flat roof, creating the most usable floor area of any loft type. Required by many London conservation areas. Almost always needs full planning.
What is and is not in the headline price
Builders quote the construction cost. Homeowners pay the total project cost. The gap is usually 15–25%.
| Item | Typical 2026 cost (London loft) | | --- | --- | | Architect fees (design 1–4) | 7–10% of build cost | | Structural engineer | £1,800–£3,500 | | Party wall surveyor (per neighbour) | £1,500–£3,500 | | Building control | £700–£1,400 | | Planning application (if needed) | £258 (householder fee) plus £1,500–£3,000 in drawings | | Independent cost expert | £2,500–£6,500 depending on scope | | Bathroom FF&E (sanitaryware, tiles) | £4,000–£15,000 | | Joinery, wardrobes, blinds | £3,000–£12,000 | | VAT on construction | 20% on the build cost |
The headline cost on a £100,000 London dormer typically becomes £125,000–£135,000 once these are added.
Where the cost actually moves
Five variables explain almost all the price difference between two loft quotes:
- Type of conversion. A mansard is genuinely twice the structural work of a rooflight.
- Staircase position and structural opening. Cutting into a load-bearing wall, or relocating an existing staircase, can add £8,000–£18,000.
- Bathroom specification. A modest en-suite is £4,000 in fittings; a walk-in wetroom with hidden cisterns and underfloor heating is £15,000+.
- Glazing. Crittall or steel-framed dormer glazing adds £4,000–£10,000 over standard aluminium.
- Party wall complexity. One difficult neighbour can add £5,000+ in surveyor fees and weeks of programme.
Common cost mistakes
- Accepting the first quote because it is the cheapest. The cheapest quote is rarely the cheapest closing out the build. See why most building quotes are wrong.
- No contingency. Hold 10–15% on lofts where the existing roof structure is unknown.
- Letting the builder choose the structural engineer. They are paid by you; they should be appointed by you.
- Skipping the party wall award. Without one, your neighbour can pause the work mid-build.
Should you appoint a cost expert?
For a single-quote loft under £80,000, a cost-checked schedule of works and a solid contract usually covers it. For any loft above £100,000, or any project quoteing to two or more builders, an independent cost expert almost always saves multiples of the fee. We typically:
- Cost-check the architect's drawings before quote.
- Issue a measured pricing document so builders price the same scope.
- Analyse the returns line by line and flag risk-loaded items.
- Value monthly payments and variations through the build.
Our extension and refurbishment cost service is built around exactly this workflow.
Related reading
- Extension cost per m² in London, the same exercise for ground-floor extensions.
- Party Wall surveyor cost (2026), the fee breakdown loft owners need.
- VAT on home building work, why almost every loft pays 20%.
Get a fixed-fee proposal for your loft project via our fee proposal form. We reply within one UK working day.
Frequently asked
- What is the cheapest type of loft conversion?
- A rooflight (Velux) conversion is the cheapest because the existing roof structure is left untouched. Expect £45,000–£90,000 nationally and £60,000–£110,000 in London, assuming the existing head height is sufficient (1.9m+ at the ridge after insulation).
- Do I need planning permission for a loft conversion?
- Most loft conversions fall under Permitted Development if you stay within the volume allowance (40m³ terraced, 50m³ semi or detached), do not extend forward of the original roof slope, and use matching materials. Mansards in conservation areas, and any loft on a flat or maisonette, almost always need full planning. Always confirm with your local authority before quote.
- How long does a loft conversion take to build?
- 6–8 weeks for a straightforward rooflight or dormer once on site. 10–14 weeks for a hip-to-gable or mansard. Add 3–4 months upfront for design, planning, party wall, and quote.
- How much do party wall surveyors add to a loft conversion?
- If you share walls with one or two neighbours, expect £1,500–£3,500 per neighbour. Three-sided terraces with awkward neighbours can reach £5,000+. Our party-wall cost guide breaks the numbers down in detail.
- Will a loft conversion add value to my house?
- Nationwide research and Land Registry data suggest a well-executed loft conversion adds 15–22% to a property's value in most UK markets, often more in London. The financial return is highest where the conversion adds both a double bedroom and an en-suite.
- Do I need a cost expert for a loft conversion?
- For a single-quote loft under £80,000, a cost-checked schedule of works is usually enough. For anything above £100,000, or where you are quoteing to two or more builders, an independent cost expert almost always saves more than the fee. See [what does a cost expert do](/homeowners/insights/what-does-a-quantity-surveyor-do).
- Is VAT charged on loft conversions?
- Yes, at 20% in almost all cases, because the work is on an existing home. The narrow exceptions (empty home over 2 years, listed building approved alterations) rarely apply. See our [VAT on home building work](/homeowners/insights/vat-on-home-building-work) guide.
