Dormer Loft Conversions: Pros, Cons & How Much They Cost in 2025

Dormer conversions are the most popular type of loft conversion in the UK, and for good reason. They give you full standing head height, proper usable floor space, and natural light through vertical windows rather than skylights in the ceiling. If you’ve been told your loft doesn’t have enough head height for a Velux conversion, a dormer is usually the answer.

What Is a Dormer?

A dormer is a structural extension that projects out from the slope of your existing roof. Think of it as a box that sits on the back (or sometimes the side) of your roof, with its own walls, roof, and windows. It creates vertical wall space and a flat ceiling inside where the roof previously sloped down to the floor.

The most common setup we build is a full-width rear dormer — running the entire length of the back of the house. This maximises floor space and gives you enough room for a bedroom, en-suite, and sometimes a small landing area.

Types of Dormer

Flat Roof Dormer

The most common and cost-effective option. The dormer has a flat roof (with a very slight pitch for drainage) covered in GRP fibreglass or a single-ply membrane. It’s not the prettiest from the outside, but since most flat roof dormers go on the rear of the house, aesthetics are less of a concern. Functionally, flat roof dormers give you the maximum internal head height.

Gable Fronted Dormer

These have a triangular front (like a mini house end) and a pitched roof. They look better than flat roof dormers and are sometimes used on the front of properties or in conservation areas where appearance matters. The trade-off is slightly less internal space due to the sloping dormer roof.

Hipped Dormer

Similar to a gable front but with the ridge set back so the roof slopes on three sides. These are the most visually sympathetic option and blend in well with existing rooflines. Common on period properties where the planning authority wants the dormer to be less visually prominent.

What Does a Dormer Conversion Cost?

In Dorset, you can expect to pay:

• Small single dormer: £35,000–£40,000

• Full-width rear dormer (most common): £40,000–£50,000

• L-shaped dormer (rear + side): £45,000–£55,000

These include the structural work, staircase, electrics, plastering, and decoration. Add £5,000–£8,000 for an en-suite bathroom, and £2,000–£3,000 for any bespoke joinery like built-in wardrobes in the eaves.

VAT is charged at the standard 20% rate on loft conversions (it’s not a new build, so the zero-rate doesn’t apply). Make sure any quote you receive states whether VAT is included — on a £45k job, that’s £9,000.

How Long Does a Dormer Take to Build?

A typical dormer conversion takes 8–10 weeks from scaffold going up to final clean. The first couple of weeks are the most intensive — cutting into the existing roof, building the dormer structure, and getting it weathertight. Once the shell is complete and watertight, the internal work (insulation, electrics, plumbing, plastering) follows a predictable schedule.

The Pros

• Maximum usable space — a full-width dormer transforms a loft from a crawl space into a proper room

• Full head height — vertical walls mean you can stand up across most of the room, not just under the ridge

• Better natural light — vertical dormer windows let in more usable light than roof windows, and you can see out of them properly

• Often permitted development — rear dormers usually don’t need planning permission (subject to limits)

• Works on most house types — terraced, semi-detached, and detached all suit dormers

The Cons

• Appearance from the rear — let’s be honest, a full-width flat roof dormer doesn’t win any beauty contests. If your rear garden backs onto a public area, this might matter

• More disruptive than a Velux — your roof is opened up for the first 2–3 weeks. There’s scaffolding, noise, and a period where the roof is temporarily weatherproofed

• Cost — roughly double the price of a Velux conversion

• Neighbours — the dormer windows overlook rear gardens, which occasionally causes friction. It’s worth having a conversation with neighbours before you start

Permitted Development Rules for Dormers

Under permitted development, you can build a rear dormer without planning permission as long as:

• The total volume added doesn’t exceed 50m³ for detached houses or 40m³ for terraced and semi-detached

• The dormer doesn’t extend higher than the existing ridge line

• The dormer doesn’t extend beyond the plane of the existing roof facing the highway (i.e., no front dormers without planning)

• Materials are similar in appearance to the existing house

• Side-facing windows are obscure glazed and non-opening (below 1.7m)

Front dormers, dormers in conservation areas, and dormers on listed buildings always need planning permission. If you’re in any doubt, we recommend applying for a Lawful Development Certificate — it costs around £100 and gives you legal proof that your dormer is permitted.

Curious whether a dormer is the right choice for your property? We’ll take a look at your loft and give you an honest recommendation — sometimes a Velux is all you need, and we’ll tell you that.