How to quote a loft conversion: structure, regs and the items that catch builders out
A loft conversion looks simple from the outside: a dormer, some stairs, a couple of rooms. The costs hide in the structure and in the building regulations, and the builders who get stung are the ones who priced it from the client's description rather than from a survey.
Survey first, price second
Measure the head height at the ridge (you want 2.2 metres or more before the floor goes up; 2.4 metres is comfortable). Check the roof structure: purlins and rafters are easy, trussed rafters mean a complete re-engineering. Look at where the stair will land and whether a bedroom below loses space. Check the water tank, the boiler flue, the soil stack, and whether the party wall is shared. Photograph everything. Then get the structural engineer involved before you price; their drawing is your take-off.
The types and what drives the price
- Rooflight (Velux) conversion: no change to the roof shape, cheapest, limited headroom
- Rear dormer: the standard terrace and semi conversion, adds most floor area
- Hip-to-gable: common on semis to make room for the stair and a dormer
- L-shaped dormer: on Victorian terraces with a back addition
- Mansard: near-complete rebuild of the roof slope, most expensive, usually needs planning permission
Most dormer conversions fall within permitted development if they are under the volume limits (40 cubic metres for terraces, 50 for others) and do not extend beyond the front roof slope; check for conservation areas and Article 4 directions. A lawful development certificate is worth the fee.
The build-up
Structure
New floor joists spanning between steel beams at the party walls or external walls, because the existing ceiling joists will not carry a floor. Steels for the dormer, the ridge and often at the stair opening. Padstones, bolted connections, temporary propping and the crane or genie lift to get steels in through the roof. This is a third of the job and the part that varies most between houses.
Roof and dormer
Strip the covering, form the dormer, insulate to Part L with the thickness that leaves you headroom, re-tile or re-slate to match, flat roof to the dormer, lead or GRP flashings, rooflights, fascia and guttering.
Stair and fire safety
Part B drives the stair design: the new floor must have a protected stair enclosure to a final exit, FD30 doors to every room off the stair (yes, including the existing ones downstairs), mains-wired interlinked smoke alarms on every floor, and an escape window in the loft rooms. Part K: a maximum 42 degree pitch, a 2 metre headroom reduced to 1.9 metres on the stair in loft conversions, and handrails. The fire doors downstairs are the item clients never expect; put them on the quote with the explanation.
Services
Electrics for the new rooms and the alarms. Plumbing to an en-suite is the usual extra: the soil connection route, the shower pump or the boiler's pressure, and the ventilation. Heating: extra radiators, and whether the boiler can cope.
Finishes
Insulated plasterboard to the slopes, skim, built-in storage in the eaves, skirtings, doors, decoration. Flooring is usually the client's item.
Items that catch builders out
- Party Wall Act notices on semis and terraces, with the surveyor's fees if the neighbour dissents
- Scaffolding with a roof tunnel over the pavement on a terrace, and the council licence for it
- Water tank removal and a conversion to an unvented cylinder or combi
- The existing ceiling: it will crack. Allow for re-skimming the rooms below
- Access for materials and waste through the house: protection and a labourer's time
- Building control inspections at each stage, and the completion certificate the client's solicitor will ask for when they sell
- Insulation thickness eating headroom: check it in the survey, not on site
Present it properly
A loft quote should be itemised by element with the structural work shown separately, the fire-safety items explained, a provisional sum for the stair design if the engineer has not finished, and the exclusions in plain English. EstiMate produces the itemised quote, the contract and the payment milestones from the same build-up, and when the client adds the en-suite half-way through, the variation is priced and approved on their phone before you order the shower.
More from the blog
- How to find construction tenders in the UK: the 2026 guide for every trade
Where UK construction tenders are actually published, what the Procurement Act changed, and how small firms in any trade can find work worth bidding for without paying an agency.
- How to price a single-storey extension in 2026: a builder's worked example
A step-by-step build-up for a 24 m² rear extension: groundworks, structure, roof, openings, services, finishes, prelims and margin, with the items builders most often forget.
- Planning applications: the lead source most builders ignore
Every approved extension, loft, conversion and new build is published by the council before the work goes out to price. Here is how to turn planning registers into a steady stream of enquiries.
- CIS explained for subcontractors and contractors: rates, returns, statements and the mistakes HMRC fines
The Construction Industry Scheme in plain English: who it applies to, the 20%, 30% and gross rates, verification, monthly returns, deduction statements, and how to reclaim what you are owed.
Ready to send better quotes?
EstiMate is free to try. Create your first professional PDF quote in minutes.
Start free — no card needed