Loft Conversion Costs in London: £250 to £300 per Square Foot, Explained

A loft conversion is the cheapest room you will ever add to a London house. It is also the one most people are quoted wrong — because the number they are handed has no working behind it. This is the honest version: what a loft actually costs per square foot, what moves that number up and down, and how to tell a real estimate from a guess.

Why the loft is the cheapest room you can add

The structure is already there. A loft conversion borrows the roof and walls you already own, instead of buying new foundations, new masonry and new groundworks. That is the whole reason it prices below every other way of extending. In London, a professionally run loft conversion typically comes in at £250 to £300 per square foot of finished works. Compare that with a rear extension at £280 to £320, or a side return at £310 to £340, and the gap is exactly the cost of the structure you are not having to build from the ground up.

What £250 to £300 per square foot actually covers

A real per-square-foot figure is not just the shell. At NU Projects it covers the structural work to open the roof, the steels and floor build-up, insulation to current regulations, the staircase, a fully finished room with electrics and heating, plastering, decoration and joinery. Where a loft starts to feel considered rather than merely converted is in the fitted joinery under the eaves — the wardrobes and storage that use the awkward angles nothing else can reach. That is craft, and it is where a good contractor earns the band.

What moves the number inside the band

Structure comes first. A simple rooflight conversion, where you keep the roofline and add windows into the existing slope, sits at the bottom of the band. A dormer, which pushes the wall out to make full-height space, sits higher. A hip-to-gable with new steelwork — rebuilding the end of the roof — sits higher again. Then the staircase, which is the design problem that decides whether the loft reads as a proper room or a compromise. Then whether you add a bathroom, which brings drainage and waterproofing. And finally the level of finish, which is the one lever entirely in your hands.

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Thinking about your own loft? Get an honest budget estimate — open book, in-house quantity surveyors, typically within seven days. No obligation, no sales pressure.

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Loft, rear extension or side return — which is right?

If you need bedrooms or a study, the loft is almost always the most cost-effective square footage in the house. If you need to open up the ground floor for kitchen and family living, a rear or side-return extension is the right tool even though it costs more per foot, because it puts the space where daily life happens. The best answer is often not either-or: a loft for the bedrooms and a rear extension for the kitchen, phased sensibly, gives the biggest lift to how a house lives and what it is worth.

Planning, permitted development and party wall

Many loft conversions fall under permitted development, which means no full planning application — but conservation areas, flats, and anything that alters the roofline can change that, so it must be checked property by property, never assumed. If your loft touches a shared wall with a neighbour, the Party Wall Act applies: it can add time to a project, but handled properly and early it cannot stop one. A contractor who coordinates building control, party wall and structure through one team keeps all of that off your inbox.

The honest caveats

A band is a range, not a quote. Head height, roof condition, conservation constraints and access all move a real project within it — and occasionally outside it. Anyone who gives you a fixed figure before construction drawings exist is guessing, and a low guess is the most expensive number in construction, because it is the one that grows. What you should expect from a contractor is not a headline price but the build-up behind it, line by line, so you can see exactly what you are paying for.

Weighing a loft against digging down instead? Read the real cost of a London basement.

Comparing routes to a bigger project? Architect versus design and build sets out both, honestly.

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Get an honest number for your loft

NU Projects is a design and build contractor on Fulham High Street, established 2010, with over 100 prime central London projects behind us. Architecture, interior design, project management and construction sit under one contract — which means one number, held from drawing to handover, and one team responsible for it.

Ready to find out what your loft would really cost? Book a consultation or call 020 7731 6841. Send us the address and we will tell you honestly what we would do with it — and what it should cost.