The word dormer gets used for two different things, and that is where most of the confusion on this topic starts. It is the name of the structure on the roof. It is also shorthand for the whole loft conversion that structure makes possible, including the floor, the stairs and the room. A quote for one is nothing like a quote for the other.

What is a dormer loft conversion?

A dormer is a structure built out from a sloping roof. It has vertical walls, a flat or pitched roof of its own, and usually a window in the front face.

The reason it is the default loft conversion in the UK is geometry. A pitched roof gives you a triangle, and most of that triangle is too low to stand in. A dormer replaces part of the slope with a vertical wall, so the ceiling is full height where the dormer sits rather than running down to the floor. You gain head height and usable floor area in the same move.

That is the difference between a dormer conversion and a rooflight conversion. A rooflight conversion, puts windows in the existing roof slope and changes nothing about the shape of the roof. It is cheaper, and it only works if the loft is already tall enough to stand up in.

What are the different types of dormer?

Is your house suitable for a dormer?

Almost any house can take a dormer. The questions that actually decide it are these.

How tall is the loft now? Measure from the top of the existing ceiling joists to the underside of the ridge. Building regulations set a 2 metre minimum ceiling height for a habitable room, but once you add a new floor structure below and insulation above, roughly 200mm to 300mm of that measurement disappears. That's why we would aim for 2.1m to avoid having to use bespoke internal doors. As a working rule, under about 2.1m to 2.2m at the ridge and a conversion gets difficult. This is a measurement your designer or a surveyor should take properly, not one to settle with a tape from the hatch.

What shape is the roof? A traditional cut roof, built from rafters and purlins on site, converts readily. A trussed roof, made from prefabricated W-shaped trusses and common in houses built from the mid-1960s onwards, fills the loft with timber that cannot simply be cut out. It can still be converted, but it needs a new structural floor and steel, and it costs more.

Is there somewhere for the stairs? The new staircase has to land somewhere with head height above it, and it usually takes space out of the room below. On many houses this is the constraint that decides the layout, and it is worth resolving before anything else is drawn.

Semi-detached houses. A semi takes a dormer as readily as a terrace, and has a larger volume allowance under permitted development, at 50 cubic metres rather than 40. Work to a semi will usually engage the Party Wall etc. Act 1996, because new steels normally bear into the shared wall.

Not sure what your roof will allow? The Extension Potential Checker gives you a first read on what your property can take.

How much does a dormer loft conversion cost?

A loft conversion where a dormer is how the space is created. This is what most people are buying. It includes the new floor structure, steels, the dormer itself, the staircase, insulation, windows, plastering and first fix electrics and plumbing.

Across the UK a main dormer runs roughly £47,000 to £80,000 excluding VAT, and an L-shaped dormer £59,000 to £90,000. London and Surrey sit at the top of both ranges, the North East at the bottom. Those regional figures are first fix benchmarks from construction tenders received through Resi, so they are a guide for early budgeting rather than a quote. Loft conversion cost sets out the full regional table and what sits inside the figures.

Want a figure for your own house? The Build Cost Calculator puts your size, location and finish level against Resi's project data.

What else do you pay for beyond the build?

None of the following sits inside the cost figures above.

Design and drawings. A measured survey of the existing house, the design, and a second more technical set of drawings for building control.

A structural engineer. Needed for the new floor, the steels and the openings in the roof. On a loft conversion this is not optional.

Building control. The application and the inspections through the build, from either the council or an approved inspector.

Planning fees, where they apply. £548 for a householder application, £274 for a lawful development certificate and planning portal admin fees.

A party wall surveyor, where the Act applies. It applies where steels bear into a shared wall, which is normal on a terrace or a semi. Where your neighbour appoints their own surveyor, the person doing the work normally bears the reasonable costs of the award. (Party Wall etc. Act 1996). How close can you build to your neighbour's boundary covers the notices and the timings.

A bat survey, where one is needed. Typically needed for alterations to roofs where the property is at risk of bats inhabiting the loft space, this is generally found near wooded areas and water.

Your fit out. Flooring, decoration, fitted storage, and the whole of any ensuite. On a loft conversion the ensuite is often the largest single item here.

Tell your insurer before work starts. Most home insurance policies restrict cover while building work is going on. Ask to see your builder's public liability and contract works cover, and check the sums insured rather than just the certificate.

Do you need planning permission for a dormer?

Usually not, if the dormer complies with permitted development requirements and the property has PD rights . Roof enlargements are covered by Class B of the Town and Country Planning Order 2015, as amended, which grants permission in law so no application is needed.

Class B does not apply, and you will need a planning application, in any of these cases:

• The house is on designated land
• The extra roof volume is over the limit
• Any part would sit higher than the highest part of the existing roof
• The dormer would sit on a roof slope that forms the principal elevation and fronts a highway
• It includes a balcony, veranda or raised platform
• The house is listed, or it is a flat or maisonette
• The materials are not similar in appearance to the existing house
• The dormer sits less than 200mm back from the eaves, measured along the roof slope (this does not apply to a hip to gable or where the dormer joins a rear or side extension)
• A side window is not obscure glazed and non-opening, unless the opening part is more than 1.7m above the floor
• An Article 4 direction removes permitted development rights for the area. Check your council’s website or map, because a direction does not appear on a plan or title deed.

A lawful development certificate is not compulsory. It is worth having anyway. At £274, it is the council confirming in writing that your dormer was inside the rules, and it is the document a buyer's solicitor asks for when you sell. A householder planning application, if you need one, costs £548 from 1 April 2026. Fees are revised each April, so confirm the current figure on the planning portal before you apply.

What do the building regulations require?

Building regulations approval is required for every loft conversion, including one that is permitted development. Permitted development is a planning route. It is not an exemption from building control, and the two are assessed by different people against different rules.

Building control checks the fire escape route, the stair, the structure and the insulation. A loft makes the house three storeys, so the stairs become a protected escape route with fire doors and interlinked smoke alarms. The stair needs 2m of headroom, or 1.9m at the centre and 1.8m at the side in a loft conversion. See staircase building regulations for every stair dimension. The new floor must carry a habitable room, usually with steels bearing into the party or flank wall. The new walls and roof must also meet the insulation standard for work to an existing home.

Does the Future Homes Standard apply? No. It was published on 24 March 2026 and comes into force on 24 March 2027, and it sets standards for newly built homes. Work to a house that already exists, including a loft conversion, sits outside it.

Resi's building regulations package covers the technical drawings and the structural calculations your builder and building control will need. Get a quote.