If you’ve started looking into a loft extension, you’ve probably already noticed the same thing everyone else does. The numbers online are all over the place. One site says £20,000, another says £150,000, and neither one tells you which figure actually applies to your house. That gap isn’t a mistake or bad reporting, it’s just how loft conversions work: the type of loft, the shape of your roof, and how finished you want the final space all change the price dramatically.
So here’s a genuinely useful breakdown of what a loft extension in London actually costs in 2026, what drives the price up or down, and how to think about budgeting for one properly.
The Short Answer
Realistically, a loft extension in London in 2026 costs somewhere between roughly £20,000 and £160,000, depending almost entirely on the type of conversion. A simple rooflight or Velux conversion tends to start around £20,000 to £45,000, a dormer conversion the most common type across London — usually runs £40,000 to £90,000, and a mansard conversion typically costs £55,000 to £100,000 or more. London projects generally run 20 to 40 percent higher than the same job elsewhere in the country, largely because of higher labour costs and tighter, trickier sites to build on.
Per square metre, most loft conversions work out to somewhere around £2,400 to £4,100, though that figure climbs in central boroughs where access is harder and skilled labour costs more.
Breaking It Down by Type
Rooflight or Velux conversions are the cheapest option because they don’t touch the shape of the roof at all — no dormer, no structural extension, just windows set into the existing roofline along with floor strengthening, insulation and a new staircase. They only work if there’s already enough headroom in the loft, generally at least around 2.1 to 2.3 metres.
Dormer conversions are the most popular type in London by a wide margin, and it’s easy to see why. They add a box-shaped extension to the rear roof slope, which creates enough extra headroom and floor space for a proper bedroom and ensuite rather than just a converted storage area. Most contractors quote somewhere between £45,000 and £75,000 for a standard rear dormer, with the final number shaped by size, finish level and how much structural work the roof needs.
L-shaped dormers cost more again, because they combine a rear dormer with a side one, usually to make room for two bedrooms or a larger single space. These tend to sit in the £55,000 to £90,000 range.
Mansard conversions are the most expensive and the most transformative, effectively rebuilding the entire rear slope of the roof at a much steeper angle to maximise usable space. They’re common on Victorian terraces and tend to cost £60,000 upward, sometimes well past £100,000 for larger or higher-spec projects.
What Actually Moves the Price Up or Down
A few things consistently make the biggest difference to the final invoice:
- Roof shape and structural work needed. Mansard and L-shaped conversions need significantly more steelwork than a simple dormer, and that shows up clearly in the price.
- Property type. Terraced houses are generally cheaper to convert than semi-detached or detached homes, mainly because there are fewer external walls involved.
- Staircase position. A new staircase that sits directly above the existing one is far simpler and cheaper than one that needs structural changes to the floor below.
- Bathroom specification. A basic ensuite adds a modest amount to the budget; a premium bathroom with underfloor heating and higher-end fittings adds considerably more.
- Planning route. Most rear dormers fall under permitted development and skip the need for full planning permission, but mansards, front-facing dormers, conservation area properties and flats almost always require a full application, which adds time and professional fees.
Don’t Forget the Professional Fees
The construction cost is usually the headline figure, but it isn’t the whole picture. Architectural design, structural engineering, and planning or building regulation applications typically add somewhere in the range of 10 to 15 percent on top of the build cost. It’s tempting to try and shave this down by going for the cheapest quote available, but this is exactly the part of the project where working with the best London architect you can find tends to pay for itself — proper structural calculations and planning-ready drawings prevent expensive redesigns further down the line, which almost always cost more than the fee saved upfront.
Why a Loft Extension Still Makes Financial Sense
Loft conversions remain one of the better-value home improvements available in London largely because they don’t eat into garden space, and they generally avoid the planning uncertainty that comes with larger ground-floor extensions. They also tend to add meaningful resale value, particularly when a loft extension London project adds a genuine extra bedroom and bathroom to a three-bedroom house, which is often the single most valuable addition a home can make.
A Realistic Way to Budget
Rather than fixating on a single number pulled from a website, it helps to work backwards from what you actually need. Decide whether you need one extra bedroom or two, whether an ensuite is essential, and how large the space realistically needs to be for your family. From there, a proper site survey and a fixed, written quote covering design, structural work, planning, and construction together will always be far more useful than any general online estimate, since every roof and every property genuinely is different.
Wrapping Up
A loft extension is one of the few home improvements in London that can add a real bedroom, a real bathroom, and real property value without touching your garden or your street-facing façade. The final cost depends heavily on the type of conversion your roof allows and the finish you’re after, but having an experienced team handle the design, structural calculations and planning together from the outset the way firms like Extension Architecture do tends to be the difference between a project that stays on budget and one that quietly creeps past it.
If you’re weighing up whether a loft extension is realistic for your house, it’s worth getting a proper survey and a written quote before settling on a figure in your head. It’s the only way to know what your specific roof actually allows.