How Much Does a Loft Conversion Cost? (UK Guide)

Most UK loft conversions come in between £35,000 and £70,000 including fees. Below that you're usually looking at a rooflight conversion on a small floor area. Above it, you're in London, in a mansard, or in a high specification.

Per square metre of usable floor area, the 2026 range runs from about £1,250 for the simplest rooflight conversion to about £2,600 for a mansard, before you adjust for region. That per-m² figure is the more useful number, because it lets you sanity-check a quote against the size of the room you're actually getting.

Region matters more than most people expect. The same dormer conversion runs roughly 35% higher in London than in the Midlands, and about 8% lower in the North of England. That's labour rates and site access, not materials, since a bag of plaster costs much the same in Leeds as in Lewisham.

Takeaway: work in cost per square metre of usable floor space, not total price. It's the only way to compare two quotes for two different-sized rooms.

Rooflight conversions, often called Velux conversions after the rooflight manufacturer, are the cheapest at £1,250 to £1,800 per m². Nothing structural changes on the roof. You're paying for a new floor, insulation, a staircase, electrics, plasterboard and finishes, plus the rooflights themselves from Velux or Keylite. The catch is that this only works if your existing head height is already good enough.

Dormer conversions run £1,500 to £2,100 per m² and are what most UK semis and terraces end up with. A box dormer built off the rear roof slope adds both headroom and floor area. It's the default recommendation from most loft companies for good reason.

Hip-to-gable conversions cost £1,700 to £2,300 per m². Where your roof slopes on the end as well as the sides, this builds that sloping hip up into a vertical gable wall, creating usable volume that simply wasn't there. Very common on 1930s semis. Pair it with a rear dormer and you're at £1,800 to £2,450 per m² for the most space a standard house can give you.

Mansards are the most expensive at £1,900 to £2,600 per m². The rear roof is rebuilt almost vertically with a shallow top slope. It gives the most space of any option and it costs the most, because it's closer to building a new storey than converting an existing one.

Takeaway: your roof shape picks the conversion type, not your budget. Find out what your roof allows before you fall in love with a price.

Take a real shape of job. A 1930s semi in Bristol, hipped roof, owners want a master suite in the loft. Usable floor area after conversion works out at 30 m². Hip-to-gable plus rear dormer, mid-range finish, en-suite, standard straight staircase, one neighbour to serve party wall notice on, and no drawings yet.

Here's the point most cost guides skip entirely, and it's the one that hurts.

A builder quotes for the building. Professional fees, building control and party wall agreements are not the builder's work, so they're not in the builder's number. On the Bristol example above they total between £6,069 and £11,718. Nobody is hiding them. Nobody volunteers them either.

Then there's the tail end. Most quotes stop at plastered and decorated-ready. Flooring, decoration, wardrobes, blinds, curtains and furniture come after the builder's van leaves. On a loft bedroom with an en-suite that's rarely under £2,000 and often closer to £5,000.

There's a third one that catches people mid-job: unexpected structural work. Older properties throw up undersized existing joists, a chimney breast that needs supporting, or a drainage run that has to travel further than anyone assumed. A contingency of 10% is the standard advice and it exists because it gets used.

Takeaway: budget the builder's quote plus 12% to 15%. If nothing goes wrong, you've saved. If something does, you haven't derailed.

Before any of this matters, measure. Take the distance from the top of your existing ceiling joists to the underside of the ridge board.

At 2.4 m or more, you're comfortable and every option is open, including the cheap one. Between 2.2 m and 2.4 m you're workable, though you'll likely need a dormer to get proper standing space where you need it. Between 2.0 m and 2.2 m things get marginal and costs climb. Under 2.0 m, you're looking at a mansard or a roof raise, and the budget conversation changes completely.

The trap is that the number you measure today is not the ceiling height you get. You lose space below to the new structural floor and insulation, and above to plasterboard from British Gypsum or similar. A loft measuring 2.3 m does not give you a 2.3 m room. Assume you'll lose 200 mm to 300 mm in total.

Takeaway: measure joist to ridge with a tape before you speak to anyone. If you're under 2.2 m, ask whether the job is worth doing before you ask what it costs.

Most loft conversions don't need planning permission. Permitted development covers rear dormers and hip-to-gable work provided you stay inside the volume allowance, which is 40 m³ for a terraced house and 50 m³ for a semi-detached or detached house. You also can't extend beyond the existing roof plane on the front elevation, and you can't go above the existing ridge. Mansards, front dormers, flats, listed buildings and conservation areas are different, and normally need a full application.

Building regulations are not optional, ever. There's no permitted development route around them.

Approved Document B, covering fire safety, is the one with the biggest cost attached. Turning a two-storey house into three storeys triggers a protected escape route, which means fire doors to every room off the stairs, mains-wired interlinked smoke alarms, and often fire-resisting floor construction. On many conversions this is a five-figure element on its own.

Approved Document K governs the staircase. The general requirement is 2 m of headroom, but loft conversions get a specific relaxation allowing 1.9 m at the centre of the stair and 1.8 m at the side. That concession is frequently the only reason a compliant stair fits.

Approved Document A covers structure, and Approved Document L sets the insulation standard, which is what quietly consumes your head height.

Get the completion certificate and keep it. Selling a house later with an unapproved loft conversion causes real problems in conveyancing, and buyers' solicitors do check.

Takeaway: ask every contractor how they've allowed for the Approved Document B escape route. The answer tells you quickly whether they've priced the job properly.

A loft conversion is one of the better-returning home improvements available in the UK, particularly where it turns a two-bedroom house into a three-bedroom one. That step moves a property into a different search bracket for buyers, which does more for value than the extra floor space alone.

Be careful with the percentage figures that circulate online. Uplift depends heavily on your street, your local ceiling price, and whether the conversion delivers a genuine double bedroom rather than a room with a bed squeezed under a slope. In an area where three-bed semis top out at a certain price, spending £80,000 on a loft can leave you above what the street will pay.

Takeaway: ask two local estate agents what your house would be worth with an extra bedroom before you commit. It's a free hour that can save a very expensive mistake.

I spoke to a builder in Manchester who's been doing loft conversions for two decades, and asked what makes him walk away from an enquiry. His answer wasn't budget. It was homeowners who've already decided what they want before anyone's been in the loft.

He described turning up to quote for a dormer, measuring 2.05 m to the ridge, and explaining that the job needed a mansard or nothing. The owners had already had two quotes for a dormer, both from firms that had priced what was asked for rather than what would work. His view was that the cheapest quote in a pile is often the one that has quietly left out the thing that makes the job possible.

He also mentioned that the single most common upsell mid-job is the staircase, because people underestimate how much floor space downstairs a compliant stair eats, and end up wanting a bespoke solution they hadn't budgeted for.

Takeaway: get someone into your loft with a tape measure before you collect quotes. A quote for the wrong conversion type is worse than no quote at all.

Rather than guess from national averages, put your own numbers in. The Loft Conversion Cost Calculator UK takes your floor area, conversion type, region and finish, then adds the extras and professional fees that sit outside a builder's quote, and gives you a realistic range plus a cost per square metre.