How to Price a Loft Conversion
Loft conversions vary more in price than almost any other job, because the type dictates the structural work. Price from the conversion type up, not from a rate per square metre.
A loft conversion can be anything from a simple rooflight room to a full mansard that rebuilds the roof, and the price gap between them is huge. Quoting a blanket rate per square metre is how builders get badly caught out — the structure, the staircase and the compliance work are what actually cost the money.
This guide covers pricing a loft conversion the reliable way: identify the type, cost the structural and building control work it forces, then add the staircase, fire safety and fit-out that every conversion needs.
Key takeaways
- The conversion type — rooflight, dormer, hip-to-gable, mansard — sets the structural cost more than the floor area.
- Structural work (steels, new floor joists, roof alterations) and building control are the big-ticket items.
- A compliant staircase and protected escape route are mandatory, and they eat into the space below.
- Fire safety — protected stairwell, fire doors, mains-interlinked alarms — is a building regs requirement, not an extra.
- It is a long job, so invoice in stages and get every variation signed as you go.
Start with the conversion type
The first question is which type of conversion the roof and the budget allow. A rooflight (Velux) conversion keeps the roof shape and just adds windows and a floor — the cheapest option, but only viable if there is already enough head height. A dormer adds a box to gain headroom and floor space. A hip-to-gable rebuilds a hipped end into a gable, and a mansard reconstructs the whole roof slope — the most involved and the most expensive.
Each type carries a different amount of structural and roofing work, which is why the type drives the price far more than the floor area does. Establish it before you cost anything else.
Structure and building control
Loft conversions almost always need new structural steelwork and a proper structural floor, because the existing ceiling joists were never designed to be a floor. That means a structural engineer's design, steels craned or carried in, and new floor joists sized to carry a habitable room. This is the single biggest cost driver on most conversions.
The work is fully notifiable and goes through building control, either on a full plans application or a building notice, with inspections at key stages. Cost the engineer, the building control fees and your time managing the inspections into the quote from the start.
Get the structural design in early
The engineer's design decides the steels, the floor and half the price. Price the conversion around the structural design, not a guess — and make clear the quote assumes it until the design is final.
Staircase, fire safety and head height
A loft is a new storey, so it needs a compliant fixed staircase — not a ladder — with the headroom and pitch the regulations demand, and that staircase has to land somewhere sensible, usually costing you a chunk of the room below. Fitting it into an existing house is often the trickiest part of the whole design.
Adding a storey also triggers fire safety requirements: a protected escape route down the stairs, fire doors to habitable rooms, and mains-powered interlinked smoke alarms throughout. These are building regs requirements, not upgrades — price them in as standard.
Fit-out, variations and staged invoicing
On top of the structure comes everything that makes it a room: insulation to meet the energy requirements, plastering, electrics and heating extended up, decoration, and often an en-suite with its own plumbing and drainage. Each is a line to price, and the en-suite in particular can be a job in itself.
A loft conversion runs for weeks, so the money must come in as you go. Invoice in stages tied to milestones — structure complete, weathertight, first fix, plastered, completion — and capture every change as a signed variation, because on a job this long the customer will change their mind.
Stage the money, sign the changes
TradePlanr lets you invoice a loft conversion in stages with card payment links and capture signed variations on site, so cash flow keeps pace with the work and extras never turn into an invoice dispute.
Frequently asked questions
Why do loft conversion prices vary so much?
Because the conversion type dictates the structural and roofing work. A rooflight conversion keeps the roof and is cheapest; a dormer, hip-to-gable or mansard involves progressively more construction. The type drives the price far more than the floor area.
Does a loft conversion need building control?
Yes. It is notifiable structural work and goes through building control on a full plans or building notice route, with inspections at key stages, plus a structural engineer's design for the steels and floor.
Do I have to fit a proper staircase?
Yes. A new habitable storey needs a compliant fixed staircase with the required headroom and pitch, plus a protected escape route, fire doors and mains-interlinked alarms. These are building regs requirements, not optional extras.
From guidance to action
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