Your roof is the largest unused space in the house. A loft conversion turns it into a bedroom, bathroom or office without eating into the garden or moving the family out for months. It is also the project most likely to surprise first-timers — not because the work is complicated, but because the decisions come early and the paperwork rarely waits. Here is how the whole thing fits together, from the first measurement to the day the carpet goes down.
Is your loft actually convertible?
Before you fall for a photo of a glass-walled dormer, check three things.
- Head height. You need roughly 2.2m of clear height beneath the ridge for a usable room, and more if you want a staircase you can walk down without ducking. Measure from the top of the ceiling joists to the underside of the ridge, not to the tiles.
- Roof structure. Traditional cut or purlin roofs usually leave more usable room than modern trussed roofs, which are criss-crossed with thin timbers. Trusses can be converted, but it often means cutting and reinforcing, which costs more.
- Where the stairs go. This decision shapes everything else. A staircase needs roughly 3m of run plus a landing, and it has to arrive somewhere sensible on the floor below — directly above the existing stairs is ideal.
If head height is tight or the roof is trussed, ask a structural engineer or an experienced loft company for a view before you spend money on drawings.
The main types of loft conversion
Almost every project falls into one of these categories, and the name usually tells you how much work is involved.
Roof light conversion
The simplest option. Windows go into the existing slope, the floor and stairs are built, and the roof line stays put. Cheapest and usually quickest, but head height in the eaves can be limited and you gain no extra floor area.
Dormer conversion
A box with vertical windows pushes out from the roof slope, giving full-height space and considerably more usable floor area. Rear dormers often fall under permitted development; front dormers, and anything on a listed building or in a conservation area, usually need planning permission.
Hip-to-gable and mansard
A hip-to-gable replaces the sloping side of the roof with a vertical wall, while a mansard lifts the whole rear slope to create near-vertical walls. Both add substantial space and cost substantially more. A mansard almost always needs planning permission.
L-shaped
Combines a main dormer with a smaller one over a rear extension. Common on Victorian terraces, and a neat way to fit in an en suite.
What it costs
Prices vary enormously by region, size, specification and how much structural work your roof needs. Rather than chasing a single number, build your budget from these parts:
- Design and engineering. Architectural drawings, structural calculations, and any planning or building control fees.
- The shell. Scaffolding, steels, floor joists, roof, windows, insulation and plasterboard. This is the bulk of the cost.
- The staircase. Frequently underestimated, and usually made to measure because the space is tight.
- Services. Electrics, heating, and plumbing if you are adding a shower or en suite.
- The finish. Plastering, flooring, doors, radiators, tiling and decoration.
As a rough shape: a straightforward roof-light conversion sits at the lower end, a dormer with an en suite sits in the middle, and a mansard or hip-to-gable with heavy structural work sits at the top. Get at least three itemised quotes and check what is excluded — scaffolding, skip hire and building control fees are common omissions. For anything involving mortgages, insurance or tax, speak to a qualified adviser rather than relying on a forum thread.
How long does it take?
Expect a straightforward job to take somewhere between six and twelve weeks on site, with larger projects running to three or four months. The preparation takes almost as long again.
- Weeks 1–6: drawings, structural calculations, planning application if needed, party wall agreements, and choosing a builder.
- Days 1–5: scaffolding up, roof opened, dust sheets over everything you own.
- Weeks 2–5: steels and floor joists in, dormer built, roof made watertight, windows fitted.
- Weeks 5–8: staircase, plumbing, electrics, insulation and plasterboard.
- Weeks 8–12: plastering, second fix, decoration, snagging and building control sign-off.
Weather slows things down, and so does a change of mind. Settle your finishes before work starts, not while the plasterer is standing there waiting.
Decisions to make early
These are the ones that get expensive to change later:
- Stairs and landing. They affect every room below, so agree the route before anything is ordered.
- En suite or not. Plumbing runs are far cheaper before the plasterboard goes up.
- Storage. Eaves cupboards behind the knee walls soak up suitcases, boxes and seasonal clutter.
- Heating and ventilation. Lofts get hot in summer. Insulate thoroughly and plan for opening windows or mechanical ventilation.
- Fire safety. Mains-linked smoke alarms, fire doors where required and a protected escape route are all part of building regulations.
Living through the work
Dust travels. Ask your builder where the access route will be, whether the stairs below will ever be blocked, and how the roof will be kept weathertight overnight. Move anything precious off the landing and out of the hallway. If the work touches a shared wall, warn the neighbours early — you may need a party wall agreement, and goodwill is easier to keep than to recover.
Getting started the sensible way
Start with a surveyor or architectural technician rather than a builder. Get drawings that show head heights and a staircase that genuinely fits, then approach three reputable loft specialists for itemised quotes. Check references, ask to see a similar project nearby, and confirm insurance and building control sign-off in writing. None of this guarantees a painless build, but it means the surprises arrive on paper, where they are cheap to fix, instead of on site, where they are not.
Photo: Didgeman / Pixabay
