The price on the quote isn't the price you'll pay

Most loft conversion budgets start with a single number someone mentions over the phone — "a dormer'll be about forty grand" — and treat that as the answer. It isn't. That figure is a headline, usually covering the structural shell and little else. A realistic budget is built up trade by trade: structure, electrics, plumbing, plastering and decoration, plus the fees nobody remembers until an invoice lands. Miss any one of those and your comfortable project becomes a stressful one.

As a rough starting point for 2024–25 pricing in the UK, a rooflight conversion tends to land between £25,000 and £45,000, a dormer between £35,000 and £55,000, and a hip-to-gable or mansard anywhere from £45,000 to £70,000+. Location, access and the age of your roof will push those numbers around considerably. Treat them as the frame, then fill in the detail below.

Structural work: the bits you never see but always pay for

This is where the real money sits, and where you have the least room to compromise. Your existing roof was never designed to carry a floor, furniture and people, so the structure has to be strengthened before anything pretty happens.

• Steel beams to carry the new floor load, often two or three, plus craning or manual lifting if access is tight. Allow £3,000–£8,000 including installation.
• Floor joists and a new floor deck, sized to meet building regulations for deflection as well as strength.
• Roof alterations — cutting rafters, forming the dormer, trimming the opening. This is the bulk of a dormer quote.
• Structural engineer's calculations, typically £500–£1,200, usually required by building control.

If your home is older or the roof has been altered before, expect more. If you share a wall, a party wall agreement may add £800–£2,000 in surveyor's fees.

Electrics, plumbing and building regulations

Services are the classic under-budgeted area. A loft is a new habitable room, so it needs its own circuits, its own heating and — if you're adding an en-suite — its own water supply and drainage.

• Electrics: sockets, lighting, a dedicated consumer unit alteration, mains-linked smoke alarms and a protected escape route. Budget £1,500–£3,500, more if you want data cabling or air conditioning.
• Plumbing: running hot and cold feeds up to the loft and, critically, getting waste down to the existing stack with adequate fall. An en-suite typically costs £2,000–£5,000 in plumbing alone before a single tile goes down.
• Heating: extending the central heating, or a separate zone. Decide early — retrofitting pipework through finished walls is painful.
• Building control fees: usually £500–£1,000, plus the cost of any inspections your contractor arranges.

Insist your quote states who is responsible for building control sign-off. If it's vague, that's a warning sign.

Plastering, decoration and the final ten per cent

Plastering is often included in a shell quote, but check the specification. Boarding and skimming a typical loft runs £1,500–£3,000. Fire-rated plasterboard on the ceiling and any wall adjoining a neighbouring property is a requirement, not an upgrade, so make sure it's named.

Decoration is the line people forget entirely. Even if you paint it yourself, you'll need to fill, sand, prime and mist coat newly plastered surfaces. If you're paying someone, allow £800–£2,500 for a loft-sized space, plus flooring, skirting, doors and furniture. Add VAT at 20 per cent to almost everything here — only certain works on empty properties qualify for reduced rates, and a loft conversion rarely does.

Your contingency: the most important line in the spreadsheet

Every experienced builder will tell you the same thing: something unexpected turns up. A rotten purlin. Asbestos in an old ceiling. A joist in precisely the wrong place. None of it is anyone's fault, but all of it costs money and time.

Set aside 10 per cent of your total budget as a minimum contingency — 15 to 20 per cent if your house is pre-1950, or if you're living in it while the work happens. Keep it genuinely separate. It is not for the nicer bathroom tiles; it is for the day the roofer finds damp. And build in time contingency too: delays of two to four weeks are normal, and if you're renting elsewhere or juggling a family around the work, that's a real cost.

Where it's worth spending more, and where to save

Spend on the things you can't change later: structure, insulation, soundproofing between floors, and getting the ceiling height right. Loft insulation must meet current standards — under-insulate and you'll be cold for the next twenty years.

Save on the finish. Standard white sockets, mid-range sanitaryware, off-the-shelf doors and a neutral carpet will all look excellent in a bright loft and cost half of a designer alternative. You can upgrade fittings in five years; you cannot easily upgrade the bones of the room.

Finally, get three itemised quotes rather than one lump sum, and compare them line by line — the cheapest overall figure is very often the one with the most gaps in it.