How Much Does a Loft Conversion Cost?

Loft conversion cost varies more by type than almost any other renovation decision — a simple roof light conversion and a full dormer or mansard conversion can differ by tens of thousands of pounds for a similarly sized roof space, because the type determines how much the roof structure itself has to change.

Typical UK loft conversion costs

These are broad, genuinely typical ranges for a full supply-and-fit conversion — actual cost depends heavily on roof type, whether the structure is altered, and specification.

• Roof light (Velux) conversion: roughly £30,000–£40,000 — no structural roof alteration, just windows, flooring, insulation, and a staircase.
• Dormer conversion: roughly £40,000–£55,000 — a genuine roof extension that adds headroom and floor space, the most common type of loft conversion in the UK.
• Hip-to-gable or full/mansard conversion: roughly £55,000–£75,000+ — the most significant structural change, often chosen for maximum usable space, frequently including an en-suite.

See our full comparison of loft conversion types for which one actually suits your roof shape and goals.

A worked example: dormer conversion with en-suite

To make the range above concrete: a mid-terrace or semi-detached house adding a single rear dormer with a double bedroom and small en-suite bathroom typically lands in the £42,000–£50,000 region — towards the lower end if the existing roof structure is sound and access is straightforward, towards the upper end if the staircase needs significant reworking or the roof timbers need remedial work once opened up. A detached house with more roof space and easier scaffolding access on a comparable dormer can sit slightly lower per square metre, since labour access is less constrained.

What a typical quote actually breaks down into

• Structural work and roof alteration: often the single largest line item on a dormer or mansard conversion — new steel or timber support, floor joist strengthening, and the roof extension itself.
• Staircase: a new, building-regulations-compliant staircase, including any knock-on work to the room it lands in.
• Windows and roof finishing: roof lights or dormer windows, external cladding or tiling to match the existing roof.
• Insulation, plasterboard and first-fix: bringing the space up to current thermal and fire-safety standards.
• En-suite plumbing (where included): a genuinely separate cost — see below.
• Electrical first and second fix: lighting, sockets, and any smoke/heat alarm requirements the loft conversion triggers for the rest of the house.
• Scaffolding: for the full duration of the project, not a one-off charge.
• Professional fees: structural engineer's calculations, and an architect or designer if you're using one — usually a few percent of the build cost, not a token sum.
• Building control fees: a fixed, relatively modest cost, but a real one to include.

What actually drives the price

Roof structure changes

The bigger the structural alteration to the roof, the higher the cost — a roof light conversion works within the existing roofline; a dormer or mansard genuinely rebuilds part of it, which means more structural engineering, more building materials, and more labour.

Staircase location

Fitting a new staircase into an existing house, without eating into a room you want to keep, is often more disruptive (and more expensive) than people expect. Where the staircase can go is one of the first things worth getting a professional opinion on.

En-suite or additional plumbing

Adding a bathroom to the new loft space means running new plumbing up through the house — genuinely additional cost, and one of the main reasons larger conversion types cost more per square metre than just the extra floor area would suggest.

Structural condition of the existing roof

Older roof timbers may need reinforcing regardless of conversion type, and this is one of those costs that's hard to confirm without a proper structural survey.

What these figures don't usually include

• VAT — confirm whether quotes are VAT-inclusive; this alone can be a meaningful difference on a project this size.
• Planning permission application fees, where planning permission (rather than permitted development) is needed.
• A structural engineer's fee for calculations, unless explicitly itemised in the quote.
• Decoration and furnishing of the finished room, which some quotes treat as a separate, later phase.
• A sensible contingency — see our general renovation budgeting guide for why building one in matters, and it matters more here than on most projects given how much can only be confirmed once the roof is opened up.

Before you get quotes: a short checklist

• Get a proper structural survey rather than assuming your roof can take a habitable room.
• Confirm whether your project needs planning permission or falls under permitted development — see our guide on planning permission for loft conversions.
• Check whether party wall notice is needed for your specific property and conversion type — see our party wall agreements guide.
• Ask each quote to itemise structural work, staircase, plumbing and finishing separately, not just give one total — it makes comparing quotes meaningfully easier.

Labour and the rest of the budget

See our loft conversion labour costs guide for how much of this figure is labour versus materials.

Timeline and avoiding common mistakes

Once you have a realistic budget, see how long a loft conversion actually takes and our guide on common loft conversion mistakes before getting quotes.