How Much Does a Loft Conversion Cost?
A span rather than a single rate, because the roof you already own decides which conversion you are able to buy.
A loft conversion runs between £1,300 and £3,000 per square metre depending on which conversion the roof will take, with the common rear dormer at £1,500 to £2,200, so around 25 square metres of usable floor is roughly £37,500 to £55,000 of building work. The head height you start with decides whether that is a rooflight conversion, a dormer or a mansard, and that choice moves the rate further than any finish does. A trussed rafter roof, standard on estate housing from the 1970s onwards, has to be replaced entirely before there is a room at all, which is a step change rather than a percentage. VAT, drawings, structural calculations, Building Control and any party wall surveyor sit outside the rate.
Between £1,300 and £3,000 per square metre, depending on which conversion the roof will actually take. The common rear dormer sits at £1,500 to £2,200, so a conversion producing around 25 square metres of usable floor is roughly £37,500 to £55,000 of building work.
VAT, the drawings, the structural calculations and the statutory fees sit outside that figure. The width of the range has almost nothing to do with the standard of the finish. It is set by the head height you start with, what the existing roof is built of, and where a staircase can be made to land.
| What decides it | Cheaper end | Dearer end |
|---|---|---|
| Head height | Rooflight conversion, roof unchanged | A dormer, or a mansard where the ridge is low |
| Roof structure | Cut roof, worked with as it stands | Trussed rafters, replaced before you own a room |
| Staircase | Off the existing landing | Turned on winders, taking more floor |
| New floor above ground | Under 4.5 metres, an escape window | Over it, a protected stairway through the house |
Head height decides which conversion you are buying
The measurement taken on the first visit is the one that fixes the rate, because it decides whether the roof has to change shape at all.
Measure from the top of the existing ceiling joists to the underside of the ridge. Where there is generous height, a rooflight conversion leaves the roof exactly as it is and prices at the bottom of the range, because nothing outside the building changes. Where there is not enough standing room over enough of the floor, a dormer buys it by building a box out of the slope, and that box has its own structure, walls, roof, cladding and windows. Where the ridge is genuinely low, a mansard or a roof raise is the honest answer, and that is a different project rather than a dearer version of the same one.
One myth is worth killing here, because it costs people a conversion they could have had. Building Regulations do not set a minimum ceiling height for a habitable room in a house. The only height genuinely fixed is the headroom over the staircase, and even that has a specific relaxation for loft conversions: 2 metres over the stairs normally, but 1.9 metres measured at the centre of the stair width, reducing to 1.8 metres at the side, is accepted where the space will not allow more. The often quoted 2.2 metres is a usability rule of thumb, not a rule.
The floor you will stand on does not exist yet
Existing ceiling joists were sized to hold up a ceiling, and that is all they will ever do.
A conversion needs a new floor: deeper joists spanning between the load bearing walls of the house, usually carried on steel beams dropped in over the existing wall plates and bedded on padstones, sitting clear of the old ceiling so nobody downstairs hears the room above. That structure, the beams, the pads and the crane or the manhandling that gets the steel up there, is the largest single item inside the rate.
What the roof is made of then makes the difference between two prices. A cut roof, built rafter by rafter on site, which is most housing up to the 1960s, has purlins and a structure that can be worked with. A trussed rafter roof, standard on estate housing from the 1970s onwards, is a web of thin timbers that cannot be cut without collapsing, so the whole structure is replaced before you own a room at all. That is a step change rather than a percentage, and it is the commonest reason a modern house converts for more than an older one.
The staircase costs floor area as well as money
The stair is joinery, a structural opening and a decision about which room downstairs gets smaller.
A compliant flight needs a run, a landing at the top and headroom over both, and it has to arrive somewhere that leaves a usable plan upstairs. On most semis it comes off the existing landing and takes a slice out of the smallest bedroom. On a terrace with a narrow stairwell it often has to turn, which needs winders and more floor. Alongside the joinery sits the work of forming the opening: trimming the existing ceiling joists, hanging them off a trimmer, and making good the landing ceiling below. None of that is visible when the job is finished, and all of it is priced.
The fire work is the item nobody budgets for
It is the thing that turns a loft conversion from a room into a job that reaches down through the whole house.
The trigger in Approved Document B is the height of the new floor above ground level, with 4.5 metres as the line, not the number of storeys. Get that wrong and you budget wrong. On a two storey house gaining a third floor, the new floor is normally above the line, and the escape route becomes a protected stairway from the loft all the way to a final exit: fire doors to every habitable room opening onto it, upgraded linings where they fall short, and linked mains powered detection through the house. Doors on the ground floor you had no intention of touching are inside that scope.
On a bungalow taking a room in the roof, the new floor may sit under the line, and an escape window can serve instead, which is a fraction of the cost. Two similar looking conversions, two different numbers, and the difference is a measurement. How the escape requirement is actually assessed is worth reading before you compare quotations.
What pushes a conversion to the top of the range
An en suite, because it brings a soil connection, falls that a loft rarely offers for free and often a pump. A hip to gable, because one end of the roof comes off and a gable wall is built up in masonry off the flank wall below, which has to be checked for the load first. Steel that cannot be lifted over the roof and has to come up the stairs in sections. And rafters too shallow to hold the insulation the regulations ask for, which forces rigid board over and under the rafters and takes head height back off you at the same time.
What the rate leaves out
Budget separately for VAT, the drawings and structural calculations, the planning route or Lawful Development Certificate, Building Control, flooring, decoration and any furniture built into the eaves. On a semi or a terrace there is also a party wall notice, because the beam ends normally bear into the shared wall, and the surveyors are paid by the building owner once a neighbour dissents. The full list of what sits inside and outside a price is on the extension costs page.
Turning the range into a figure
Four things convert this into a price: the height under the ridge, whether the roof is cut or trussed, where the stair can land, and how far above ground the new floor sits. All four are settled in an hour with a tape and a look through the loft hatch, which is why a rate quoted over the phone is worth what it costs.
The type by type breakdown, from a rooflight conversion through to a mansard, sets out what each one buys and what it asks of the roof. These figures are indicative for the Colchester area at the review date below. Use them to sense check a quotation rather than as one.
Last reviewed August 2026.
Thinking about the job itself?
Most people who call us already have drawings and permission and want a builder to price the work and build it. Either way the survey and the written quotation cost nothing.