DORMER LOFT CONVERSIONS

What a dormer actually does

A dormer is a box built out from the existing roof slope, with vertical walls and a flat or shallow roof of its own. The point of it is headroom. In an unconverted roof the only place you can stand up is a narrow strip along the ridge; everything either side slopes away to nothing. A dormer replaces part of that slope with a vertical wall and a level ceiling, so the usable floor area — the area where an adult can stand — often more than doubles.

That is why a dormer is the default answer on the Victorian and Edwardian terraces in Roath, Canton, Cathays and Grangetown. Those roofs generally have height at the ridge but very little usable width, and a rear dormer is the most direct way to convert one into a bedroom you can put a wardrobe in.

Where it goes

• Rear dormer — by far the most common. Built on the back slope, out of sight from the street, and generally the version that falls within permitted development.
• Side dormer — used where the rear slope is short or already occupied by an outrigger.
• Front dormer — rare, because anything extending beyond the roof plane facing a highway falls outside permitted development and needs a full application.
• L-shaped dormer — wraps around into the rear outrigger, common on the larger Victorian terraces and the version that yields the most space.

Planning permission

A rear dormer is often permitted development, subject to the volume allowance — commonly around 40 cubic metres of added roof space on a terraced house and 50 on a semi or detached. The conditions matter as much as the volume: materials similar in appearance to the existing house, nothing extending beyond the roof plane on the highway elevation, no balconies or raised platforms, and side-facing windows obscure-glazed and non-opening below 1.7m.

Those rights are removed in conservation areas, on listed buildings and where an Article 4 direction applies. Cardiff has a number of conservation areas, so it is worth checking your address on the council's map before the design is settled. Whether or not planning is needed, Building Regulations approval is required either way.

Cost and timescale

Published UK guides put dormer conversions at roughly £30,000 to £60,000 in 2026, with the midpoint for a standard rear dormer around £50,000 for about 30 square metres. Those are national averages weighted by London, not our quotes — see our loft conversion cost page for the full breakdown and what moves the figure.

On site, a straightforward rear dormer on a traditional cut roof more commonly runs six to eight weeks, with the noisy structural work concentrated in the first fortnight. Add lead-in time before that for drawings, structural calculations and Building Control, and longer again where a planning application is needed.

Things worth knowing early

• A full-width dormer gains the most space but is the most visible; some streets carry a planning expectation of a set-back from the party walls and the eaves.
• Flat dormer roofs are usually finished in EPDM rubber or GRP fibreglass. Both are sound; the detail at the abutment matters more than the material.
• The dormer cheek walls are a common cold-bridge and condensation point if the insulation detail is rushed.
• If your roof is trussed rather than cut, the structural work comes before the dormer and changes the price.

Related pages

• Loft conversions in Cardiff
• Hip-to-gable loft conversions
• Velux & rooflight conversions
• Mansard loft conversions
• Loft conversion costs
• Planning permission & building regs

Thinking about your loft?

A short site visit tells us the roof type, the head height and where the stairs can land — which is most of what decides the design and the price. We will tell you if a conversion is not the right answer for your house.