Ask five London builders what a loft conversion costs and you will get five very different answers, often before anyone has looked at your roof. That is because the price depends far more on the house than on the conversion type. GMAS does not publish a price list: every loft is quoted in writing after a survey, and the price is agreed before work starts. What we can do is explain what moves the number, so you can read quotes properly and compare them fairly.
The types of loft conversion
Rooflight conversion
Roof windows are fitted into the existing slope and the floor, stairs and insulation are added, with no change to the roof shape. It needs the least structural work, but only suits lofts that already have enough head height.
Rear dormer
A box-shaped extension built out from the rear slope, giving full head height across most of the new room. It is the most common loft in London terraces and semis.
L-shaped dormer
A dormer on the main roof joined to a second one over the back addition, typical of Victorian terraces. It usually creates two bedrooms or a bedroom with a bathroom.
Hip-to-gable
On a semi or end-of-terrace with a sloping side roof, the hip is built up into a vertical gable wall, often combined with a rear dormer.
Mansard
The rear slope, and sometimes the front, is rebuilt almost vertically with a flat top. It gives the most space and is common in conservation areas, but it is the biggest structural job and usually needs planning permission.
What decides the cost of a loft conversion
The type and size
Volume drives cost. A mansard involves rebuilding most of the roof; a rooflight conversion leaves it in place. Within each type, a larger floor area means more structure, insulation, plaster and finishes.
The existing roof structure
Older cut-timber roofs are easier to open up than modern trussed roofs, which need more steel to replace. The engineer's design and the amount of steelwork are among the biggest variables.
Head height and the floor
Where there is not enough height, the ceiling below may need lowering or the roof raising, both of which add work. New floor joists sized for a bedroom are always needed.
The staircase
Where the new stair lands decides how much the floor below has to change. A stair that fits over the existing one is simpler than one that needs a landing reconfigured or a room shortened.
Bathrooms and plumbing
An en suite adds plumbing, drainage, waterproofing and fittings. A soil stack that is far from the bathroom position adds pipework, and a boiler that cannot supply another bathroom may need upgrading.
Fire safety and escape
Building Regulations require a protected escape route, usually fire doors to the rooms off the stair and mains-wired interlinked smoke alarms, sometimes a sprinkler or misting system where the layout needs one. In a house that has never been altered, this work reaches every floor.
Windows, finishes and specification
Window type, built-in storage, flooring, lighting and bathroom fittings can make two lofts of the same size very different in price.
Access, scaffold and London factors
Scaffold, skips, parking suspensions and permits, congestion charge zones and narrow streets all affect cost, and they vary from street to street across London.
Dormer or mansard: which suits your house?
On most London terraces the choice comes down to a rear dormer or a mansard. A dormer is quicker, involves less structural work and is often permitted development. A mansard gives more usable space and a more traditional look from the street behind, and is the form many councils prefer in conservation areas, but it almost always needs planning permission and rebuilds more of the roof. The right answer depends on your street, what your neighbours have built and how much space you need.
Does a loft conversion add value?
In most of London an extra bedroom, and especially a bedroom with its own bathroom, makes a house more valuable and more saleable. How much depends on the street, the quality of the finish, how the stair works with the floor below and whether the paperwork is complete. A loft without a Building Control completion certificate can cause problems when you sell, so the sign-off matters as much as the build.
The costs outside the build
- Design: measured survey and drawings.
- Structural engineer: calculations for the steels, joists and any wall changes.
- Planning: an application fee, or a lawful development certificate if the loft is permitted development.
- Building Control: plan approval and inspections, ending in a completion certificate.
- Party wall: notices to neighbours and, if they appoint surveyors, the surveyors' fees.
- Making good: decorating the floor below where the stair arrives.
Ask every builder which of these are included. A low headline figure often leaves several of them out.
Planning permission and permitted development
Many lofts on houses can be built as permitted development, within an extra volume allowance of 40 cubic metres for a terraced house and 50 for a semi or detached house, with no dormer facing the road and materials that match. Flats and maisonettes do not have these rights, and in conservation areas roof extensions need planning permission. Much of inner London is in a conservation area, so check before you design. A lawful development certificate confirms a permitted loft and is worth having when you sell. Every loft needs Building Regulations approval whether or not it needs planning.
Lofts in flats and conservation areas
Top-floor flats can have loft conversions, but there is no permitted development for flats, so planning permission is always needed, and the freeholder's consent is usually required under the lease because the roof space is rarely part of the flat. Allow for a licence for alterations and the freeholder's costs. In conservation areas, councils often have design guidance on roof extensions, such as preferring mansards with traditional windows, setting dormers back from the eaves or keeping the front roof slope untouched. Reading that guidance before the drawings are prepared saves redesign later.
Listed buildings
Any change to a listed building's roof needs listed building consent as well as planning permission, and the materials and details will be closely specified.
The Party Wall Act
Steels are usually set into the party wall, so on a terraced or semi-detached house you will normally need to serve notice on the neighbours under the Party Wall etc. Act 1996 before work starts. Allow time for this in the programme.
How long a loft conversion takes
A straightforward dormer is typically several weeks on site, with mansards and lofts with bathrooms taking longer. Design, planning and party wall notices add time before work starts. Most of the work happens from scaffold, so you can usually stay in the house until the stair is cut through.
Living with the work
Most of a loft conversion is built from scaffold, so for much of the job the house stays usable. The disruptive stages are breaking through for the stair, fire safety work on the floors below, and connecting plumbing and electrics, which involve dust and short shutdowns. Planning those stages, and keeping the household informed of dates, makes the whole job easier to live with.
Questions to ask before you sign
- Who is my single point of contact, and how often will I get updates?
- Who arranges the engineer, Building Control and party wall notices?
- What happens if the existing structure turns out different to the drawings?
- How are changes priced and approved?
- What certificates will I receive at the end?
How to compare loft conversion quotes
- Make sure every quote prices the same drawings and engineer's design.
- Check what is included: steels, fire doors, alarms, windows, bathroom, decorating, scaffold and skips.
- Look for provisional sums, which are allowances rather than prices, and ask what they cover.
- Ask how changes are priced and approved during the job.
- Ask who manages Building Control and the completion certificate.
How GMAS prices a loft
- Survey. We look at the roof structure, head height, stair position and access.
- Design and engineering. Drawings and calculations, or we price yours.
- Written fixed quote. Itemised, agreed before work starts, with any changes quoted and approved in writing.
- Build. One point of contact manages every trade, from steels to decorating.
- Sign-off. Building Control completion certificate and any electrical certificates handed over.
See our loft conversion page, or loft conversions in south east London.
Frequently asked questions
How much does a loft conversion cost in London?
It depends mainly on the type, the size, the existing roof structure, the stair position and whether you add a bathroom. We quote every loft in writing after a survey.
Which loft conversion is the cheapest?
A rooflight conversion usually needs the least structural work, but it only works where there is enough head height. Mansards involve the most work.
Do I need planning permission for a loft conversion?
Not always. Many house lofts are permitted development, but flats and houses in conservation areas need planning permission. Every loft needs Building Regulations approval.
What is not usually included in a loft conversion quote?
Design, engineer's fees, planning or certificate fees, Building Control, party wall surveyors and decorating below are often extra. Ask each builder to list them.
How long does a loft conversion take?
A straightforward dormer is typically several weeks on site, longer for mansards and lofts with bathrooms, plus time for design, approvals and party wall notices.