How much does a loft conversion cost in London in 2026?

The type of loft decides most of the price before any choice of finish. Rooflights only keeps the roof as it is and adds windows in the slope: the least structure and the smallest room. A rear dormer builds a box out of the back slope for full head height across most of the floor. An L-shaped dormer adds a second dormer over the back addition of a Victorian or Edwardian terrace. A hip-to-gable conversion, for a semi or end-terrace with a sloping side roof, builds the side up into a gable wall. A mansard replaces the back slope with a near-vertical wall and a flat top, the closest thing to adding a storey.

The table gives the all-in total for each type on a typical two-storey house at three finishes, with the floor area assumed. Each range is wide on purpose: the low end is a plain loft on an easy house, the high end a harder one with more to put right.

Measured on the building work alone, a rear dormer with a good-quality finish works out at about £2,400 per m² of new floor, in a range of £1,800 to £3,000. A mansard is about £2,850 per m², because the roof is rebuilt rather than cut into.

Per square metre is a weaker guide for a loft than for an extension. Much of the cost is fixed whatever the size of the room: scaffolding, the staircase, the steels, fire protection and building control. A small loft spreads those over fewer square metres, so its rate looks high even when its total is the lowest. Budget from the totals; use per m² only to compare quotes for lofts of a similar size.

It depends on which loft it is for and what it covers. A contractor's quote is usually for the building work only, often without VAT, while the totals above also carry the fees and a contingency. For a rear dormer with a good-quality finish, compare a quote with the building work figure of £50,000 to £84,000, after adding VAT if it says "plus VAT".

Then look at the type and the finish. A rear dormer's all-in total runs from £57,000 to £105,000 at a straightforward finish to £76,000 to £140,000 at a high-end one. Moving the same dormer from good quality to high end adds about £7,000 to the likely figure; dropping to a straightforward finish saves about £14,500. Adding an en suite adds about £4,000. An L-shaped dormer is £77,000 to £139,000 and a mansard £107,000 to £186,000 at the same finish, so a quote that looks high for a dormer can be ordinary for a mansard.

If a quote sits above the top of the range for its type and finish, ask what it covers that the range does not: work to the floor below, a larger dormer, better windows, or a provisional sum for something not yet seen. If it sits well below the bottom, ask what has been left out. Either way, keep the contingency of 12% of the build, about £6,000 to £10,000 for a rear dormer, on top.

The building work covers the structure, the new stairs, the roof work, insulation, windows, and the first and second fix of electrics, plumbing, plaster and joinery, ready to decorate. The totals then add the fees for a rear dormer: architect or designer £4,000 to £11,000; structural engineer £1,000 to £2,500; lawful development certificate £359 with the Planning Portal charge; building control £1,000 to £1,500; party wall surveyors £0 to £6,000. Together the fees come to £6,500 to £21,500. The party wall line is zero if the neighbours consent in writing.

For a rear dormer on a typical house outside a conservation area, the planning answer is that it is normally permitted development, subject to the limits checked in the report: roof volume allowance (GPDO 2015 Sch. 2 Pt 1 B.1(d)); not on the front (GPDO 2015 Sch. 2 Pt 1 B.1(c)); set back from the eaves (GPDO 2015 Sch. 2 Pt 1 B.1(b), B.2(b)); side windows (GPDO 2015 Sch. 2 Pt 1 B.2(c)). A lawful development certificate confirms it in writing. A conservation area or an Article 4 direction changes that street by street, so check with the council. The council fees are in the box below.

Outside the totals: an en suite or a new bathroom, structural changes to the floor below, furniture, carpets and blinds, and any time living elsewhere while the work is done.

Build costs move with parking, access and labour. On the London baseline a rear dormer with a good-quality finish is £63,000 to £116,000. In Barking and Dagenham, Bexley and Havering, the lowest-cost end of the scale, it is £58,000 to £109,000. In Kensington and Chelsea and Westminster, the highest, it is £72,000 to £132,000.

The borough changes the building work, not the statutory fees or the planning rules. Each borough's page, linked below, gives the totals for every loft type there.

The staircase. A new flight has to land on the floor below without losing a bedroom, with fire protection along the escape route. A tight landing can mean a winding stair made to measure.

The steels. Most dormers need new steel beams to carry the floor and the dormer walls. More steel, or a crane to lift it, adds cost, and every beam needs a structural engineer's calculation.

Head height. With too little height under the ridge, the choices are lowering the ceilings below or raising the roof. Both cost more, and raising the ridge normally needs a planning application, so check with the council.

Dormer size. A full-width dormer costs more than a small one, and an L-shaped dormer more again, as the table shows.

An en suite and the finish. Plumbing at the top of the house, tiling and sanitaryware add about £4,000 at a good-quality finish, and the move from good-quality to high-end fittings, windows and joinery is the biggest choice you control.

Loft conversion costs, the planning route and a timeline in each London borough.

Barking and Dagenham · Barnet · Bexley · Brent · Bromley · Camden · Croydon · Ealing · Enfield · Greenwich · Hackney · Hammersmith and Fulham · Haringey · Harrow · Havering · Hillingdon · Hounslow · Islington · Kensington and Chelsea · Kingston upon Thames · Lambeth · Lewisham · Merton · Newham · Redbridge · Richmond upon Thames · Southwark · Sutton · Tower Hamlets · Waltham Forest · Wandsworth · Westminster

Is a loft conversion cheaper than a rear extension?

Not always in total, but it usually gives more floor for the money. A rear dormer of about 28 m² is £63,000 to £116,000 all in at a good-quality finish, about £2,400 per m² for the building work. A single-storey rear extension of about 17.5 m² is £61,000 to £114,000, about £3,750 per m². The extension has foundations, drainage and a lot of glass; the loft reuses the walls and roof already there.

How long does a loft conversion take?

About 23 to 39 weeks from first sketch to sign-off for a loft conversion with a rear dormer, of which 6 to 10 weeks are on site. Design, the council's decision and the building regulations drawings take most of the rest.

Do these loft conversion prices include VAT and fees?

Yes. Every figure includes VAT at 20%, and the all-in totals include the professional and council fees, £6,500 to £21,500 for a rear dormer, and a contingency of 12% of the building work at a good-quality finish (10% at straightforward, 15% at high end). The contingency is money to have available, not a cost you will certainly pay.