Loft Conversion Cost Calculator UK
Loft conversion quotes vary wildly, and it's rarely obvious why. Two builders can look at the same roof and come back £20,000 apart. This calculator gives you a realistic figure before the quotes land, based on the things that actually move the price: the type of conversion, your floor area, where you live, and the finish you're after. It covers rooflight, dormer, hip-to-gable and mansard conversions using 2026 UK build rates, and it adds the costs that quotes routinely leave out — professional fees, building control and party wall agreements.
Adds headroom and floor space. The default choice on most UK semis and terraces.
Measure the usable floor area, not the full footprint. Space with under about 1.5 m of head height doesn't count as usable room.
Good quality fittings, the usual assumption in a quote
Adds £4,000–£7,000
A standard straight flight is included in the base rate. Tick this only for a bespoke, spiral or space-saver stair. Adds £1,500–£3,500.
Architect / design, structural engineer, building control. Uncheck only if you already have drawings and approvals.
Terraced = usually 2. Semi-detached = usually 1. Detached = 0. Adds £1,000–£2,000 per neighbour.
Typical cost
£57,500
Likely range: £46,000 – £69,000
Cost per m²
£2,300
| Item | Low | High |
|---|---|---|
| Base build cost | £37,500 | £52,500 |
| En-suite bathroom | £4,000 | £7,000 |
| Party wall (1 neighbour) | £1,000 | £2,000 |
| Architect / design fees | £2,500 | £5,500 |
| Structural engineer | £500 | £1,000 |
| Building control | £600 | £1,000 |
| Total | £46,000 | £69,000 |
How to use this calculator
- 1. Pick the conversion type. If you don't know yet, start with dormer. It's what most UK semis and terraces end up with, and it sits in the middle on price.
- 2. Measure the usable floor area. Enter length × width, or switch to area mode if you already know the m². Only count space with real head height. A loft with 30 m² of floor plan might give you 20 m² you can actually stand up in.
- 3. Set your region and finish. Region is the single biggest swing after conversion type. London runs about 35% above the Midlands baseline; the North of England runs about 8% below.
- 4. Add the extras that apply. An en-suite is the most common addition and the most commonly forgotten in a first budget.
- 5. Leave professional fees switched on. They're real money and they're almost never in a builder's quote. Switch them off only if you already have your drawings and approvals in hand.
Takeaway: the two inputs worth getting right are usable floor area and region. Everything else moves the number by less.
What each conversion type costs
| Conversion type | Typical £/m² (2026) | Typical total at 25 m² | Planning | Best for |
|---|---|---|---|---|
| Rooflight / Velux | £1,250–£1,800 | £31,000–£45,000 | Usually permitted development | Lofts that already have head height |
| Dormer | £1,500–£2,100 | £37,500–£52,500 | Usually PD if rear-facing | Most UK semis and terraces |
| Hip-to-gable | £1,700–£2,300 | £42,500–£57,500 | Usually PD | Hipped roofs on semis and detached |
| Hip-to-gable + rear dormer | £1,800–£2,450 | £45,000–£61,000 | Usually PD | Maximum space on a 1930s semi |
| Mansard | £1,900–£2,600 | £47,500–£65,000 | Usually required | Terraces where nothing else gives the space |
Totals shown are at the Midlands baseline with a mid-range finish, before extras and fees.
The gap between a rooflight and a mansard on the same footprint is roughly £20,000. That's not a finish decision, it's a structural one, and it's usually decided by your existing roof rather than your preference.
Takeaway: conversion type is set by your roof shape and head height far more than by budget. Price the type your roof allows, not the one you'd like.
Head height decides everything, and no calculator can fix it
This is the check to do before you think about money. Measure from the top of the existing ceiling joists to the underside of the ridge board.
| Available height (joist to ridge) | What it means |
|---|---|
| 2.4 m or more | Comfortable. Most conversion types work, rooflight is viable |
| 2.2 m to 2.4 m | Workable. Expect a dormer to get usable standing space |
| 2.0 m to 2.2 m | Marginal. Dormer or mansard, and costs climb |
| Under 2.0 m | Needs a mansard or a roof raise. Budget at rebuild level |
You lose height from both ends once work starts. Insulation and the new floor structure eat into the bottom; plasterboard and any structural work eat into the top. A loft that measures 2.3 m today does not give you a 2.3 m room.
Takeaway: measure joist to ridge before you get a single quote. If you're under 2.2 m, the conversation changes from "which type" to "is this worth doing at all".
The building regulations that shape the cost
A loft conversion is fully notifiable work, and the requirements drive real cost, not just paperwork.
Approved Document B (fire safety) is the expensive one. Converting a two-storey house into three storeys triggers a protected escape route: fire doors to every room off the stair, mains-wired interlinked smoke alarms, and often a fire-resisting floor construction. On many jobs this is a five-figure line on its own.
Approved Document K (stairs) requires 2 m headroom over a staircase, but loft conversions get a specific relaxation: 1.9 m at the centre of the stair width and 1.8 m at the side is acceptable. That relaxation is often the only reason a stair fits at all.
Approved Document A (structure) covers the new floor joists and any steels. Approved Document L sets the insulation standard, which is what quietly steals your head height.
The Party Wall etc. Act 1996 applies if you're cutting into or building off a shared wall, which covers almost every terrace and semi. You serve notice on your neighbours, and if they dissent you pay for surveyors.
Takeaway: fire safety under Approved Document B is the regulation most likely to surprise you on price. Ask any builder quoting how they've allowed for the protected escape route.
Worked example: a dormer conversion on a Leeds semi
A three-bed 1930s semi in Leeds. The owners want a master bedroom with an en-suite in the loft. Usable floor area measures 25 m². Rear dormer, mid-range finish, standard straight staircase, one neighbour to serve party wall notice on, and no drawings or approvals yet.
- Base build: 25 m² × £1,500–£2,100 = £37,500–£52,500
- Mid-range finish (× 1.00): unchanged
- North of England (× 0.92): £34,500–£48,300
- En-suite: +£4,000–£7,000
- Party wall, one neighbour: +£1,000–£2,000
- Architect and design (7%–10% of base build): +£2,415–£4,830
- Structural engineer: +£500–£1,000
- Building control: +£600–£1,000
Range: £43,000–£64,000. Typical: £53,500. That's about £2,140 per m².
The spread is wide, and it's honest. The bottom of that range is a local builder with straightforward access and a modest spec. The top is a specialist loft firm with a full design service and a high-end bathroom. A single quote of "£48,000" tells you nothing about which end you're standing at until you've read what's in it.
Takeaway: get three written quotes and compare what's included, not the headline figure. On a job this size the inclusions swing the price more than the builder's margin does.
A point most cost calculators skip
Almost every loft conversion cost guide gives you a build figure and stops. The build figure is not the cost.
Professional fees, building control and party wall agreements sit outside a builder's quote almost universally, because they aren't the builder's work to do. On the Leeds example above they add £3,500 to £6,800. Nobody hides them, but nobody volunteers them either, and homeowners routinely budget the quote and then find another £5,000 landing over the following two months.
There's a second one that's easier to miss. Most quotes exclude decoration, flooring, wardrobes and blinds, because those come after the builder leaves. On a loft bedroom with an en-suite, that's rarely under £2,000 and often nearer £5,000.
Takeaway: budget the quote plus 12% to 15% for the things that sit outside it. If you come in under, that's a good surprise rather than a bad one.
Related tools
- Stud Wall Calculator UK — timber and plasterboard for the partitions in your new loft room
- Plasterboard Calculator UK — sheets and screws for boarding out the conversion
- Rafter Length Calculator UK — work out rafter runs for a dormer or gable
- Roof Pitch Calculator — find your existing pitch before assessing head height
- Driveway Cost Calculator UK — 2026 supply-and-fit rates for the drive
- How Much Does a Loft Conversion Cost? UK Guide — the full breakdown behind these figures
Frequently asked questions
Disclaimer: This calculator gives cost estimates for planning purposes only. Actual loft conversion prices vary with roof structure, access, existing head height, structural work required, drainage runs, specification and local labour rates. Building regulations requirements referenced here follow current England guidance (Approved Documents A, B, K and L); Scotland, Wales and Northern Ireland differ. Always obtain at least three written quotes from insured contractors, and consult a structural engineer and your local building control body before committing to any work. BuildByJai is not a construction contractor and accepts no liability for budgets set on the basis of these figures.