Loft conversion cost calculator
A loft conversion costs £22,000 to £32,000 for Rooflight / Velux; £40,000 to £58,000 for Dormer and £55,000 to £75,000 for Mansard / hip-to-gable. Set the options below to price your own, adjusted for where you live.
Work out what a loft conversion costs. Pick the type of conversion, tick an en-suite if you want one, and this calculator gives you a price range for your region. Built from our own guide prices, not quotes, so treat it as a sanity check before you get anyone out.
Loft conversion cost calculator
Set it up to match your job. Everything updates as you change it, and nothing is sent anywhere.
Built from the same figures as our loft conversion guide: £22,000 to £32,000 for a rooflight conversion, £40,000 to £58,000 for a dormer and £55,000 to £75,000 for a mansard or hip-to-gable, with an en-suite adding £3,500 to £7,000. The price covers structural work, new floor joists, a staircase, insulation, plastering, electrics and heating, the windows or dormer construction, and building control sign-off. Regional figures use the same multipliers as every other page here. What it cannot tell you is whether your loft converts at all. The usual test is head height: you want roughly 2.2 to 2.4 metres from the existing floor to the underside of the ridge before work starts, because the new floor comes up and the insulated ceiling comes down. A trussed roof, common in houses built from the 1960s onwards, needs the trusses cutting and replacing with steels, which is a real job rather than a detail.
Loft conversion cost by region
| Type of conversion | UK average | London | North / Scotland / Wales |
|---|---|---|---|
| Rooflight / Velux | £22,000 to £32,000 | £28,500 to £41,500 | £20,000 to £29,500 |
| Dormer (most common) | £40,000 to £58,000 | £52,000 to £75,500 | £37,000 to £53,500 |
| Mansard / hip-to-gable | £55,000 to £75,000 | £71,500 to £97,500 | £50,500 to £69,000 |
Guide prices for loft conversion, before any extras. UK guide prices reviewed June 2026, not quotes. Set the calculator above to your own job to add the extras.
The three types, and why the price triples between them
A rooflight conversion changes nothing structural about the roof. You put in a proper floor, a staircase, insulation and Velux windows in the existing slope. It is the cheapest by a wide margin at £22,000 to £32,000, and it is only possible if you already have the head height and a roof shape that gives you usable floor area.
A dormer builds outwards from the roof slope to create a box with a flat roof and full-height walls. That is what most people picture, it is the most common conversion in the UK, and at £40,000 to £58,000 it costs roughly twice a rooflight because you are building a structure rather than fitting out a space.
A mansard or hip-to-gable rebuilds a substantial part of the roof, replacing a sloping side with a vertical wall. It gives the most space and it is the most expensive at £55,000 to £75,000. It is also the one most likely to need planning permission rather than falling under permitted development.
Will your loft actually convert
Head height decides it. Measure from the existing loft floor to the underside of the ridge. You want about 2.2 to 2.4 metres before work starts, because the finished floor will rise by 100 to 200mm to take new joists, and the insulated ceiling will drop. Building regulations require 2 metres of head height over the staircase, and that is usually the binding constraint rather than the room itself.
The other thing to look at is what is holding your roof up. Older houses tend to have a cut roof, with purlins and rafters and an open middle, which converts easily. Houses built from roughly the 1960s onwards usually have W-shaped trusses filling the whole space. Those can be cut out, but only after steel beams take the load, and that adds cost and time.
What a loft conversion does to the floor below
The staircase has to come from somewhere, and on most houses that means taking a chunk out of a bedroom or a landing. It is the part people forget when they compare a loft conversion with an extension: you are adding a room upstairs and losing floor area below. A conversion that turns a three-bed into a four-bed but leaves an unusable box room has not really added a bedroom.
It is worth drawing the staircase before anything else. Building regulations set the maximum pitch and the minimum head height over it, so there is less freedom than you might expect, and the answer sometimes changes which type of conversion makes sense.
Planning permission and party walls
Many rooflight and rear dormer conversions are permitted development within the volume limits, which for a house is 40 cubic metres for a terrace and 50 for a semi or detached. Anything at the front of the house facing a road, anything in a conservation area, and most mansards will need planning permission. Building control is required either way.
If you live in a terrace or a semi, you will almost certainly need a party wall agreement, because the steels supporting the new floor bear into the shared wall. Budget roughly £700 to £1,500 per neighbour and start the process early, since the notice periods are fixed and cannot be compressed.
Common questions
How much does a loft conversion cost in the UK?
About £22,000 to £32,000 for a rooflight conversion, £40,000 to £58,000 for a dormer and £55,000 to £75,000 for a mansard or hip-to-gable, with £45,000 the typical figure. An en-suite adds £3,500 to £7,000. London runs about 30% above the UK average and the North, Scotland and Wales about 8% below.
What head height do I need for a loft conversion?
Roughly 2.2 to 2.4 metres from the existing loft floor to the underside of the ridge before any work starts. The finished floor rises 100 to 200mm for new joists and the insulated ceiling drops, so you lose height from both directions. Building regulations require 2 metres over the staircase, which is usually the tighter constraint. Below about 2.2 metres you are into lowering ceilings or raising the roof, both of which change the project completely.
Is a loft conversion cheaper than an extension?
Usually, for the same amount of space, because the roof and the foundations already exist. A dormer conversion at £40,000 to £58,000 typically buys more floor area than the same money spent extending. The trade-off is that you lose space below for the staircase, you are limited by the roof you have, and a loft room cannot be a kitchen or a family living space in the way a ground floor extension can.
Do I need planning permission for a loft conversion?
Often not. Many rooflight and rear dormer conversions are permitted development within the volume limits of 40 cubic metres for a terrace and 50 for a semi or detached. You will need permission for anything facing a road at the front, anything in a conservation area, and most mansards. Building control approval is required in every case, permitted development or not.
How long does a loft conversion take?
About 6 to 10 weeks on site for a typical dormer, plus design, planning and party wall notices beforehand, which can add two to three months. A rooflight conversion is quicker at around four to six weeks. The house stays liveable throughout for most of it, though the week the roof is opened up is noisy and the staircase going in is disruptive to the floor below.
These are independent guide prices based on typical UK jobs in 2026. Your actual cost depends on your property, spec, access and where you live. Always get at least three written quotes before committing. These are independent Costwise guide prices, reviewed June 2026, not quotes and not a survey of real quotes. A real price depends on access, condition and what the job actually involves, so use this to sanity-check what you are told and always get three written quotes.