Most loft conversions are priced after twenty minutes in a roof space with a torch, a tape and a customer standing on the landing below asking whether it’ll fit an en-suite. By the time the structural engineer’s calculations land, the steel has grown, the stair has moved, and the number you gave is a number you’re stuck with.

The fix isn’t padding. It’s knowing which few things actually decide the price, measuring those properly before you quote, and putting the rest in as labelled allowances the customer agrees to in advance. Every figure below is illustrative — your roof, your access and your labour set your numbers.

Establish which type of conversion it actually is

Customers say “loft conversion” and mean four very different jobs. Settle this first, because the type sets the scaffold, the roof works and the planning position.

Rooflight conversion. The roof shape doesn’t change — you’re strengthening the floor, fitting rooflights and lining out. Cheapest by a wide margin, and only possible where the existing height is generous.
Rear dormer. A flat-roof box built out of the rear slope to buy headroom and floor area. Adds a structure, a flat roof, cheek walls, glazing and guttering — and usually turns an unusable loft into a proper room.
Hip-to-gable. The hipped end built up square, nearly always paired with a rear dormer. Scaffold goes all round, the gable needs tying in and matching in render or brick, and there’s more roof to re-form.
Mansard. The whole rear slope rebuilt at a steep pitch. Biggest space, biggest job, most planning-sensitive.

Permitted development, planning and party wall all turn on the type and the property. Don’t advise on it — tell the customer to confirm the position with their local authority and the guidance on gov.uk, and say in the quote who applies and pays.

Measure the four things that decide the price

Head height

Measure from the top of the existing ceiling joists to the underside of the ridge, in several places, and note where the roof closes in. New floor structure builds up from below and insulation plus plasterboard comes down from above, so the finished room is always shorter than your tape says today. There’s a minimum headroom over the new staircase set by the Building Regulations — check the current figure with Building Control before you commit, because a loft that misses it either becomes a dormer job or stops being a job.

The existing roof structure

The single biggest price fork. A traditional cut roof — rafters, purlins, struts, a sensible amount of clear space — converts readily. A modern trussed rafter roof, with W-shaped webs filling the space, can’t simply have those webs cut out: the roof has to be re-supported first, which is a different order of structural work and cost. Identify which you’ve got before you say a figure out loud.

Where the staircase lands

The one that catches people out. The new flight has to start somewhere on the floor below, at a complying pitch and headroom, and land where there’s height to stand up. In practice that often means taking a chunk out of the back bedroom, or losing a cupboard and part of a landing. Fire safety comes with it — a protected route, doors, alarms, sometimes an escape window — which is a Building Control conversation, not a guess.

Sketch the stair position on the survey and get it agreed in writing. “We didn’t realise we’d lose that bedroom wall” is not a conversation to have after the steel’s in.

What else lives up there

Cold-water storage tank, header tank, pipework across the joists, the cables feeding the upstairs lights, a chimney breast in the middle of the floor plan, a soil vent pipe coming through. Then the covering: age of the tiles and battens, whether there’s felt under them, the state of the stack, valleys and flashings. While the scaffold is up is the only sensible time to re-roof — price that as a separate option so the customer decides, rather than absorbing it quietly.

Price it in stages

Build the quote around the stages you’ll actually work in. It makes the estimate more accurate and gives you a natural payment structure.

Preliminaries. Scaffold erect, hire and dismantle — priced by week, and loft jobs overrun — plus dust and floor protection to the stairs and landing, skips and permits.
Structural opening-up. Strip-out, removing the covering where the dormer or gable goes, temporary support.
Steel and new floor. Beams, padstones, bearings cut into the party and external walls, new joists, deck. Include the labour of physically getting beams into the roof — through a roof opening, up the scaffold, occasionally craned.
Dormer or gable construction. Framing or blockwork, cheeks, flat roof deck and membrane, trims, glazing openings.
Roof covering, leadwork and rainwater goods. Making good into the existing roof, flashings, new guttering runs.
Staircase. Forming and trimming the opening, the flight, balustrade, and making good to the room you took it out of.
First fix. Electrics, heating, en-suite plumbing and the waste route down to an existing stack.
Insulation, fire separation and plasterboard — signpost Building Control on the regulated elements rather than specifying from memory.
Second fix and finishes. Doors, architrave, skirting, sanitaryware, decoration, flooring, eaves storage if specified.
Building Control sign-off and snagging.

An illustrative build-up for a rear dormer with an en-suite — your numbers will differ:

Preliminaries & scaffold (8 weeks)        £4,200
Strip-out & structural opening-up         £2,600
Steels, padstones & new floor             £7,800
Dormer construction & flat roof           £9,400
Roof making good & leadwork               £3,100
Staircase & making good below             £4,600
First fix electrics, heating, plumbing    £3,300
Insulation, fire separation & plaster     £5,200
Second fix, sanitaryware & decoration     £6,100
Chimney works (prov. sum)                 £1,400
Existing roof repairs (prov. sum)         £1,200
Contingency                               £2,100
                                         -------
                                          £51,000
                               (+ VAT if registered)

Put the real unknowns in as provisional sums

Steel sizes come from the engineer, not from you, and they often come back heavier than the one you had in mind. Chimney stacks look fine from the ground and are soft at the haunching. Nobody knows the state of the battens until the tiles come off. These are textbook cases for a provisional sum: a labelled allowance inside an otherwise firm quote, adjusted against actual cost once the item is exposed or specified. The method for pricing around what you can’t see applies to lofts more than to most jobs.

Write the basis next to each one — “allows for two 203mm beams per the preliminary sizing; adjusted against the engineer’s final schedule, re-priced and agreed in writing before ordering”. When a provisional sum does move, handle it as a proper written variation rather than a verbal mention on site; the discipline for agreeing extras as you go is what stops a £900 steel upgrade becoming a final-account argument.

Then list the exclusions on the face of the quote, not in small print:

Structural engineer’s calculations and Building Control fees
Planning application or lawful development certificate, and any party wall award and surveyor’s fees
Asbestos survey and removal — textured coatings and old insulation board are common in lofts
Consumer unit upgrade or rewiring beyond the new circuits
Fitted wardrobes, blinds, carpets and furniture unless listed
Re-roofing the existing roof, unless taken as the stated option
Relocating a boiler or cylinder, unless listed

Stage the money and present it properly

On a job this size you should never be more than one stage out of pocket, and payments should hang off milestones you can both see rather than dates that slip.

Milestone	Payment
On acceptance, before scaffold and steel order	10%
Scaffold up, structure opened, steels in	25%
Dormer built and weathertight	25%
Staircase in, first fix and plaster complete	25%
Sign-off and snagging agreed	15%

Present it as a document, not a figure. The customer will show it to a partner and possibly a mortgage adviser, and the quote that wins is usually the one that explains itself: stage costs, provisional sums flagged with their basis, exclusions where they can’t be missed, the payment schedule, an outline programme in weeks, how long the price holds, and your insurance and guarantee position. The anatomy of a quote that gets accepted covers that structure, and for anyone running loft and extension work regularly the builder’s side of quoting is where the day count and the sub-trades decide the margin.

This is general guidance, not legal advice — planning, party wall and Building Regulations positions vary by property, so check gov.uk and your local authority for the current requirements.

The takeaway

A loft conversion is priced on four facts: which type it is, how much height you’ve genuinely got, whether the roof is cut or trussed, and where the staircase lands. Measure those before you give a number. Then stage the quote, put the steel, the stack and the existing roof in as labelled provisional sums with a stated adjustment method, exclude the statutory fees in writing, and tie the money to visible milestones.

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How to Quote a Loft Conversion