Dormer loft conversion cost in the UK (2026 guide)

A dormer is a box built out from the roof slope to make headroom and floor space. The label covers very different jobs, though, and the price follows the job rather than the label. This guide goes past the basics in our loft conversion cost guide and looks only at dormers: the rear box, the L-shaped dormer over an outrigger, and the front dormer that usually needs planning permission.

Do we give a price for a dormer loft conversion?

No, and here is why. Which? says conversions that need structural work, such as a dormer, cost more than a rooflight conversion, and that a large dormer conversion will cost almost twice as much as a small conversion, using BCIS 2025 data (Which?, loft conversion costs 2026, updated 24 April 2026, read 12 October 2026). The pound tables are for members. We checked council, GOV.UK and Planning Portal pages and found no dated public figure for the build itself, so we do not print one. Cost pages that quote a confident range with no source are guessing.

What we can do is show which choices move the price and give you every published fee you will pay on top, each with its date.

The three kinds of dormer

The flat-roof rear dormer

This is the standard: a box on the back slope with a flat roof, vertical cheeks and windows in the rear wall. Government guidance accepts felt, lead or zinc on the flat roof of a dormer, because it is seldom seen, but says the face and sides should look similar in colour and design to the main roof as seen from the ground (GOV.UK, permitted development technical guidance, updated 10 September 2019). Which? describes a dormer as an extension that protrudes from the slope to create more headroom, and says it suits almost any house with a sloping roof (Which?, how to plan a loft conversion).

The main cost question for a rear dormer is width. A box that stops short of the party walls is a smaller structure than one that runs full width, and Which?'s point that a large dormer conversion costs almost twice as much as a small conversion is the reason to ask for the dimensions on every quote.

The L-shaped dormer over an outrigger

Many Victorian and Edwardian terraces have a lower back addition, often called an outrigger. An L-shaped dormer wraps the main roof and the outrigger roof into one continuous enlargement, which is how you get a second bedroom and a bathroom rather than one room. The government guidance says an enlargement can join the original roof to the roof of a rear or side extension even where the pitches differ, and it lets that enlargement go beyond the outer face of the original wall where it joins the two (GOV.UK technical guidance).

That flexibility is for the planning test only. The structure is a different matter: the junction between two roofs at different levels needs its own steel and careful weatherproofing, and the outrigger walls must be able to carry what they now hold. Ask whether the structural engineer has seen the outrigger walls, not just the main roof.

The front dormer

Front dormers are where planning rules bite. In England the permitted development guidance says dormers on a principal elevation that fronts a highway are not permitted development and need a planning application. Rooflights on that slope may still be allowed. A "principal elevation" is the one that usually contains the main entrance, and a house on a corner can be read in a different way, so ask the council if you are unsure.

A householder application in England costs £548 from 1 April 2026 (GOV.UK, planning fees from 1 April 2026). Outside England the process and fees differ, so use your council's own pages. Do not read the fee as a sign of the cost of the build. It is one small line, and a refusal costs you the design fees as well.

Planning limits that shape a dormer

For a rear dormer on a house in England, the permitted development route has several tests that catch designs by surprise. They come from the GOV.UK technical guidance, which applies to England, and your council can remove them with an Article 4 direction.

• Volume. Added roof space, counting any earlier enlargement, may not exceed 40 cubic metres for a terraced house or 50 cubic metres for any other house. The guidance defines a terrace house as one in a row of three or more, so an end-of-terrace is in the 40 cubic metre group.
• Height. No part of the house may be higher than the highest part of the existing roof, which means the main ridge.
• Set-back. The enlargement should be at least 0.2 metres from the original eaves, measured along the slope, so far as practicable.
• Wall line. The new work may not stick out past the outer face of the original walls, except where it joins the roof of a rear or side extension.
• Side windows. Any window in a side wall or side roof slope must be obscure-glazed and non-opening unless the opening part is more than 1.7 metres above the floor.
• No balcony. Verandas, balconies and raised platforms are excluded.
• Designated land. Conservation areas, National Parks, the Broads, areas of outstanding natural beauty and World Heritage Sites are excluded, and so are flats.

If a design sits near a limit, a lawful development certificate puts the council's view in writing. It has its own fee, lower than a full application; check the exact amount on the Planning Portal or your council's fee page before you apply. Wales and Scotland have their own routes: in Scotland you need a building warrant and should check roof alterations with your council.

Structure and steel

Which? says you must consult a structural engineer, and that the engineer checks the loft is suitable and designs the changes. For a dormer the engineer is deciding three things: how the new floor is carried (usually new joists or beams that bear onto the walls), what carries the roof where you cut the slope open, and how the load reaches the foundations.

Roof age matters more than most owners expect. Which? says homes built before about 1970 usually have open, self-supporting rafters that are easier to convert, while later homes often have roof trusses, which are trickier to convert and can double the cost (Which?, loft conversion costs 2026). That is the biggest single swing we can source, and it is a reason to give every builder the same roof photos before they quote.

Ask three questions about steel: who designed it, whether the calculations go to building control, and whether the steel supply and fixing are in the price or an allowance.

Party walls and scaffolding

On a terrace or semi, new steel often bears on the shared wall. The GOV.UK explanatory booklet lists cutting into a party wall to take the bearing of a beam, "for example for a loft conversion", among the work that needs notice. It also says the Act is separate from planning and building regulations, so having approval for those does not remove the duty to serve notice. The building owner usually pays for the award (GOV.UK, Party Wall etc. Act 1996 booklet, published 12 May 2016). Serve notice at least two months before the start.

Scaffolding is a quote line that people forget. If it stands on the pavement, the council usually wants a licence. Solihull's page, for example, lists £274 for the first month and £87.50 for each extra month from 1 April 2026, and says a licence is needed for a scaffold on a public highway, including a rear lane (Solihull Council). Other councils charge differently, so treat Solihull as one example, not a norm.

Fire escape and headroom

A dormer makes a new storey, and that is a fire safety event. Approved Document B, volume 1 says that where a conversion creates a storey above 4.5 metres, the whole escape route must be addressed, with fire-resisting doors (minimum E 20) and partitions (minimum REI 30), including upgrading existing doors. The alternative is sprinklers to open-plan areas, plus a fire-resisting partition and door that give access to a first-floor escape window (GOV.UK, Approved Document B vol 1, paragraphs 2.21 and 2.23, read 12 October 2026). This is where "just add a dormer" turns into a bill for doors on your landing.

Headroom is the other test. Which? gives the minimum as 2.2 metres across at least 50% of the floor area (Which?, regulations, 24 April 2026). Approved Document K asks for 2 metres of stair headroom, with a reduced 1.8 metres allowed for loft conversions where there is not enough space (GOV.UK, Approved Document K, 2013 edition).

Building control fees for a dormer

Councils publish their own schedules, and the spread is useful. The Royal Borough of Windsor and Maidenhead lists a loft conversion without a dormer (maximum 60 square metres) at a £432 plan charge and £480 inspection charge, and one that includes a dormer or changes the roofline at £432 and £660, all VAT-inclusive, from 1 April 2026 (RBWM). Dover lists a loft with dormers up to 60 square metres at £300 plus £660, with no date shown on the page (Dover District Council). Read your own council's page, and see our loft conversion cost guide for the Oxford and Leeds schedules.

Which? gives a typical time on site of closer to five weeks for a dormer. Treat that as the builder's programme to test, not a promise.

Rear dormer or hip to gable?

If your house has a hipped side, a hip to gable conversion is often combined with a rear dormer to make one room. That job has its own structure and approvals, so we cover it in our hip to gable loft conversion cost guide.

Comparing dormer quotes

Ask each builder to state the dormer width, depth and height, the roof type they found, the steel schedule, the stair position, the fire doors and who pays for scaffolding licences, party wall surveyors and building control. A quote that is silent on those is not a lower price, it is a smaller promise.

You can search for a loft conversion builder or read about loft conversions first. We do not state what builders on this site charge, because the directory is still being filled town by town. Confirm insurance and scheme membership on the scheme's own site before you pay a deposit.