Attic conversion Cost Calculator

How this calculator works

Every figure in this calculator is the one published in the cost table on our attic conversion guide. Pick the option that matches your job and the calculator returns that published range, adjusted for your region - it does not model anything the guide does not already say.

That is deliberate. A calculator whose rates drift from the page it sits under is worse than no calculator, because two numbers on the same site disagree.

What it cannot know: access, the condition of what is already there, and anything structural. Those are what separate three real quotes from each other.

Why two lofts on the same street quote differently

What’s holding your roof up decides the job. A traditional cut roof – rafters and purlins cut on site – leaves an open void that’s reasonably straightforward to convert. A trussed roof, the factory-made W-shaped frames on most houses built since the 1960s, fills the space with timber that’s doing structural work. You can’t just cut it out; new beams have to take over the load first, and that’s the single biggest reason two lofts nearby land on very different numbers.

Head height is the first thing to measure. As a rough rule of thumb you want around 2.2m from the existing floor to the ridge for a comfortable conversion. Less than that and you’re into lowering the ceiling below or raising the roof, both of which shift the budget bracket. Measure before you fall in love with the idea.

The staircase has to land somewhere, and it eats floor on the storey below as well as up top. Building regs also want a protected escape route – usually a fire door onto a stair enclosure that runs down to a final exit – so you may end up upgrading doors on a landing you already have.

The old ceiling joists were sized to hold up plasterboard, not people, so the floor gets strengthened with new joists spanning between the walls. Insulation goes in at the same time, between and over the rafters or in the new floor, to meet current thermal standards.

How you get light and headroom in is the big design fork. Rooflights sit flush in the slope and are the lighter-touch, cheaper option – the “Velux loft” end of the cost table. A dormer or a hip-to-gable change adds standing room and floor area but costs more, which is the spread you can see across the published options above.

Every loft conversion needs building control sign-off, whatever the planning position – the structure, fire safety and insulation all get inspected and certified. You can read what that approval covers on the government’s building regulations guidance. Keep the completion certificate; you’ll want it when you sell.