How to Use the Loft Conversion Cost Calculator
- Choose your units. UK mode uses £ and m²; USA mode uses $ and sq ft.
- Choose the conversion type (UK) or the scope of work (USA).
- Enter the usable floor area of the finished room or rooms.
- Choose your region (UK) or local cost factor (USA).
- Set the allowance for extras. UK headline prices often exclude VAT, fees, party wall costs, an en-suite and finishes; the calculator adds 20% by default.
- Optional checks: enter your loft's head height, and the size of a planned dormer, to see whether the conversion is likely to work and to fit within permitted development.
How Much Does a Loft Conversion Cost in the UK?
Typical 2026 costs for a 3-bed house, by type:
| Conversion type | Outside London | London | Per m² (outside London) |
|---|---|---|---|
| Rooflight (Velux) | £30,000–£40,000 | £40,000–£55,000 | £1,800–£2,400 |
| Rear dormer | £45,000–£65,000 | £60,000–£90,000 | £2,200–£2,800 |
| Hip-to-gable (with rear dormer) | £55,000–£75,000 | £75,000–£100,000 | £2,400–£3,000 |
| L-shaped dormer | £60,000–£80,000 | £75,000–£105,000 | £2,500–£3,200 |
| Mansard | £65,000–£90,000 | £85,000–£120,000+ | £2,800–£3,800 |
London prices typically run 30–50% above the rest of the UK.
Which type is right for you?
- Rooflight (Velux): the cheapest, with no change to the roof shape. Only works if the loft already has good head height.
- Rear dormer: a box-shaped extension out of the back roof slope, giving lots of full-height floor space. The most common type.
- Hip-to-gable: replaces a sloping side (hipped) roof with a vertical gable wall. Common on semi-detached and detached houses, often combined with a rear dormer.
- L-shaped dormer: two dormers joined over the main roof and a rear outrigger. Popular on Victorian terraces.
- Mansard: rebuilds the rear roof slope almost vertically. It gives the most space but usually needs planning permission.
Regional differences
| Region | Typical rear dormer cost (3-bed semi) |
|---|---|
| London | £60,000–£95,000 |
| Home Counties | £55,000–£80,000 |
| Birmingham, Manchester, Bristol, Leeds | £45,000–£65,000 |
| Wales, North East, North West (outside cities) | £40,000–£58,000 |
| Edinburgh and Glasgow | £45,000–£68,000 |
| Northern Ireland | £35,000–£55,000 |
Worked example: rear dormer outside London
A rear dormer creating 22 m² of floor space, in a regional city:
- Build cost: 22 × £2,200–£2,800 = £48,400–£61,600
- Plus 20% for VAT, fees, party wall and finishes: about £58,000–£74,000
The same conversion in London would be about £67,800–£86,200 before extras.
Is My Loft Suitable? Head Height
The first thing to check is head height. Measure from the top of the ceiling joists to the underside of the ridge beam at the highest point. As a rule of thumb:
- 2.3 m or more: good. Most conversion types are possible, including a rooflight conversion.
- 2.2–2.3 m: tight but often workable, usually with a dormer.
- Under 2.2 m: difficult. You may need to lower the ceilings below or raise the roof, which is expensive and needs planning permission.
Roof structure matters too. Traditional cut-timber roofs (with rafters and purlins) are easier to convert. Trussed roofs with W-shaped timbers, common in houses built since the 1960s, need more structural work and often steel beams.
Planning Permission and Permitted Development (England)
Many loft conversions don't need a planning application, because they're permitted development. The main limits are:
- Volume: extra roof space of no more than 40 m³ for terraced houses and 50 m³ for detached and semi-detached houses, including any earlier roof extensions
- No higher than the highest part of the existing roof
- No dormers on the front roof slope facing a road
- Materials similar in appearance to the existing house
- No balconies, verandas or raised platforms
- Side-facing windows must be obscure-glazed and fixed shut below 1.7 m from the floor
- Dormers set back at least 20 cm from the original eaves, except for hip-to-gable work
Permitted development rights don't apply to flats and maisonettes, and are removed or restricted in conservation areas, National Parks and some other areas. Mansard conversions usually need planning permission. A Lawful Development Certificate (£274 from April 2026) confirms your conversion is permitted development; a full householder planning application costs £548.
Estimating dormer volume: a box dormer is roughly a wedge, so its volume is about width × height × depth ÷ 2. A dormer 6.5 m wide, 2.4 m high and projecting 3.5 m is about 27 m³, well within 40 or 50 m³. The calculator includes this check.
Building Regulations for Loft Conversions
Every loft conversion that creates a habitable room needs Building Regulations approval, even if planning permission isn't needed. The main requirements:
- Structure: new floor joists, and often steel beams, designed by a structural engineer
- Fire safety: a protected escape route down the stairs to the front door, fire-resisting doors to habitable rooms off the stairway, and mains-wired, interlinked smoke alarms
- Stairs: a permanent staircase, with at least 2 m headroom (reduced to 1.9 m at the centre of the stair in loft conversions)
- Insulation: the roof slopes, dormer walls and cheeks must meet current Part L standards
- Sound insulation between the new floor and rooms below
If you share a wall with a neighbour, you'll usually need to serve a Party Wall Act notice, and possibly pay for a party wall surveyor.
Attic Conversion Costs in the USA
In the USA, finishing or converting an attic typically costs:
| Scope | Cost per sq ft | Typical total |
|---|---|---|
| Basic finish (insulation, drywall, flooring) | $30–$60 | $5,000–$30,000 |
| Living space (bedroom or office) | $60–$100 | $8,000–$30,000 for a bedroom |
| Full conversion with plumbing or structural work | $100–$150+ | $20,000–$80,000+ |
Common extras:
| Item | Typical cost |
|---|---|
| Code-compliant permanent stairs | $3,000–$10,000 |
| Egress window (for a bedroom) | $2,500–$6,000 |
| Dormer to add headroom | $8,000–$30,000 |
Per-sq-ft rates understate the cost of small attics, because stairs, egress and structural work cost about the same whatever the size.
Worked example (USA): attic bedroom
A 550 sq ft attic converted to a bedroom at $60–$100 per sq ft:
- Finish: $33,000–$55,000
- Plus new stairs and an egress window: about $38,500–$71,000
US code requirements for attic rooms
Under the International Residential Code (IRC), as adopted locally:
- Ceiling height: at least 7 ft over at least half of the required floor area; areas under 5 ft high don't count towards the room's floor area
- Room size: habitable rooms generally need at least 70 sq ft and at least 7 ft in any direction
- Egress: every sleeping room needs an emergency escape opening, usually a window with at least 5.7 sq ft of clear opening and a sill no higher than 44 inches
- Stairs: a permanent stair; pull-down ladders don't count for living space
- Floor structure: attic joists are often sized only for storage and may need strengthening
You'll need a building permit, and your local building department will confirm which code edition applies.
Does a Loft Conversion Add Value?
Industry research suggests a well-executed loft conversion can add around 15–20% to a home's value, especially when it turns a 3-bed house into a 4-bed with an en-suite. It's often cheaper per m² than a ground-floor extension and doesn't use garden space. Compare the numbers with the Home Extension Cost Calculator.
How to Budget and Plan
- Add 15–25% to headline quotes for VAT, architect and engineer fees, party wall costs, an en-suite and finishes.
- Allow time: a rear dormer typically takes about 8–12 weeks on site, plus 3–6 months for design, approvals and party wall agreements.
- Pay in stages. Pay no more than about 10% up front, and hold back at least 5% until the Building Regulations completion certificate is issued.
- Plan the stairs early. Where the new staircase goes often decides how the whole floor below is laid out.
- Insulate properly: see the Insulation Calculator. For new rooflights and dormer windows, the Window Replacement Cost Calculator can help with budgets.
Frequently Asked Questions
How much does a loft conversion cost in the UK?
Roughly £30,000–£40,000 for a rooflight conversion and £45,000–£65,000 for a rear dormer outside London, and 30–50% more in London. Mansards cost £65,000 or more.
How much does a dormer loft conversion cost per m²?
About £2,200–£2,800 per m² outside London for a rear dormer.
What is the minimum head height for a loft conversion?
About 2.2–2.3 m from the top of the ceiling joists to the underside of the ridge.
Do I need planning permission for a loft conversion?
Often not, if it's within permitted development limits: 40 m³ for terraced houses or 50 m³ for other houses, with no dormers facing the road. Building Regulations approval is always needed.
How long does a loft conversion take?
A typical rear dormer takes about 8–12 weeks on site, plus several months of design and approvals.
How much does it cost to finish an attic in the USA?
About $30–$60 per sq ft for a basic finish and $60–$100 for a bedroom, plus stairs, egress and any dormer.
Does a loft conversion add value?
Often around 15–20%, especially when it adds a bedroom and bathroom.
Get quotes from loft conversion specialists
A survey of your roof decides what's possible. Compare at least three quotes.