Loft conversion: costs, types and hiring the right firm

A loft conversion adds a bedroom under your roof; get a structural engineer and planning check done before hiring a builder.

LC-01What a loft conversion actually involvesA loft conversion turns your roof space into usable floor space, usually a bedroom with an en-suite. The simplest type just adds windows into the existing roof slope. Bigger jobs build out the roof itself to create more headroom, which costs more and takes longer. Which type suits your house depends on the roof shape and the ceiling height you already have.

LC-02Check whether you need planning permissionMany loft conversions fall under 'permitted development' and don't need planning permission, but there are limits on size, materials and how far the roof can project. You will usually need full planning permission if you're in a conservation area, live in a flat or maisonette, or have already used up your permitted development allowance with a previous extension. HomeGraft has a 'Know the rules' guide on planning permission and permitted development. Don't take a builder's word for this — check with your local council's planning department before you commit to anything.

RULES

KNOW THE RULES

Planning permission may be required depending on your roof design, your property type and any previous extensions. Confirm with your local planning authority before starting work.

LC-03Hire a structural engineer before you hire a builderA structural engineer works out whether your existing floor joists and walls can take the weight of a new room, and designs the steel beams and new staircase. Get this done before you approach builders, because their quotes should be based on the engineer's drawings, not guesswork. Typical cost is £500 to £1,500 for the survey and structural design. Skipping this step is the single biggest cause of disputes and cost overruns later.

LC-04Party Wall Act: check if you need to notify neighboursIf you share a wall with a neighbour — semi-detached or terraced houses — and the work cuts into that wall, for example to rest steel beams on it, you must serve a Party Wall notice at least two months before that work starts. This is a legal requirement, not just good manners, and ignoring it can lead to delays or legal action later. HomeGraft has a 'Know the rules' guide on the Party Wall Act.

RULES

KNOW THE RULES

Under the Party Wall Act 1996, you have a legal duty to serve notice on affected neighbours before certain work starts, and to follow the formal process if they object.

LC-05Submit a Building Regulations applicationA loft conversion always has to meet Building Regulations, covering structural strength, fire escape, insulation and staircase safety. You can apply via 'full plans', where drawings are approved before work starts, or a 'building notice', where an inspector checks the work as it happens. Most loft conversions use full plans because the structural work is significant. Your builder or architect usually handles this submission, but it's your legal responsibility to make sure it's done.

RULES

KNOW THE RULES

Building Regulations approval is a legal requirement for loft conversions. Work done without it can cause problems when you sell the house.

LC-06Get quotes from loft conversion specialistsGet at least three quotes from builders who specialise in loft conversions, not general builders. A proper quote should itemise structural work, windows, staircase, insulation, electrics, plumbing and plastering separately, and state what's excluded. It should also show the payment schedule, start date, and how long the job will take. Be wary of a single lump sum with no breakdown — it hides where your money is actually going.

LC-07Questions to ask and red flags to watch forAsk to see photos of their last three loft conversions, a reference you can actually call, and proof of insurance and any trade accreditation. Ask who applies for Building Regulations approval and who serves the Party Wall notice — it should be clear, not vague. Red flags include wanting full payment upfront, no written contract, no fixed start date, and reluctance to put the structural engineer's drawings into the quote. A builder who avoids these questions is a builder to avoid.

LC-08Realistic costs and timescalesFor an average three-bedroom house outside London, a simple roof-window conversion typically costs £35,000 to £45,000, a dormer conversion £45,000 to £65,000, and a hip-to-gable or larger conversion £55,000 to £75,000 or more. London and the South East usually run higher than these figures. Costs climb towards the top of the range with en-suite bathrooms, larger dormers, complex roof shapes, or if structural work turns out more extensive than first thought. Expect 2 to 3 months for design and approvals, then 6 to 14 weeks on site depending on the type of conversion.

LC-09Managing the build once it startsPayments should be staged against progress, not handed over in advance — for example on completion of structural work, then first fix, then final finish. Building Control will inspect the work at key stages, such as before the floor is covered and once insulation is in. Keep a simple written record of any changes agreed on site, since verbal changes are where disputes start.

LC-10What commonly goes wrongThe most common problems are: starting building work before planning or Building Regulations approval is confirmed, underestimating the cost of the staircase and losing space downstairs for it, and discovering the roof structure is weaker than expected once work has started. Disputes also happen when the quote didn't clearly separate what's included, so extra costs appear partway through. Budgeting a contingency of around 10-15% on top of your quote is sensible for this reason.

LC-11Final sign-off and paperworkOnce work is finished, you should receive a Building Regulations completion certificate — keep this safely, as buyers' solicitors will ask for it when you sell. If a new bedroom changes your home's size or layout significantly, your home insurance provider should also be told. Check your EPC (Energy Performance Certificate) is updated if required, since an added bedroom affects it.

RULES

KNOW THE RULES

A Building Regulations completion certificate is the legal proof the work meets required standards. Keep it; you will need it when you sell your home.

LC-12If you live in Wales, Scotland or Northern IrelandIn Wales, the planning permission and Building Regulations system is broadly similar to England, but check with your local authority as some rules differ slightly. In Scotland, you need a building warrant from your local council before work starts, instead of a Building Regulations application, and this must be approved before construction begins. In Northern Ireland, building control is handled by district councils, and planning rules on permitted development differ from England, so always check locally before hiring anyone.

RULES

KNOW THE RULES

Scotland requires a building warrant before work starts, approved by the local council. Rules in Wales and Northern Ireland also differ from England — check with your local authority.

KNOW THE RULES

Before you start: Do I need planning permission for a loft conversion? · Do I need a party wall agreement?

Save this as a project

Back to all guides