What the job involves
A loft conversion turns unused roof space into a proper room, usually a bedroom, office or bedroom with en suite. The work is much more than boarding the floor and fitting a window. The space has to be structurally sound, safe to escape from, insulated and reachable by a permanent staircase.
The two most common routes are:
- Velux (rooflight) conversion: roof windows are fitted into the existing roof slope. The roof shape stays largely as it is, so there is less structural work. It suits lofts that already have enough headroom.
- Dormer conversion: a box-shaped extension is built out from the roof slope to add floor area and standing room. It costs more and takes longer, but it gives a more usable room.
Other types, such as hip-to-gable and mansard conversions, change the roof shape more and are bigger jobs. This guide concentrates on Velux and dormer work.
The usual sequence is a survey, drawings and structural calculations, then building regulations and any planning checks. After that come scaffolding, steel beams and floor strengthening, then the roof alterations or dormer build. First fix electrics and plumbing follow, then insulation, plastering, the staircase, second fix and decorating. Building control inspects at set stages and signs the work off at the end.
What affects the price
No two lofts are the same, which is why quotes vary so much. These are the factors that move the price most.
- Dormer or Velux: a dormer involves building a new structure, with more materials, more labour and more weatherproofing. Rooflights only are generally the cheaper route where headroom allows.
- Size and shape of the roof: a larger roof means more scaffolding, more insulation, more plasterboard and more labour. A complicated roof with hips, valleys or chimneys adds structural work.
- Existing structure: older roofs built with cut timbers often need more alteration than modern trussed roofs, which have to be reworked with steel. The condition of the existing timbers and ceiling joists also matters.
- Floor strengthening: ceiling joists were rarely designed to carry a bedroom. The more the floor needs reinforcing, the higher the labour and materials.
- Staircase position: the stairs need adequate headroom and must fit without ruining the room below. Moving walls or reworking the landing adds cost.
- Bathroom or en suite: this brings in plumbing, waste routes, extraction, waterproofing and tiling. Running a waste pipe a long way, or needing a pump, pushes the price up.
- Windows and finish: the number, size and type of windows, and whether you want special glazing, a Juliet balcony or bespoke joinery, all change the total.
- Fire safety: building regulations require suitable means of escape, fire-resistant doors and interlinked smoke alarms. Upgrading doors and altering the floors below can add work.
- Access and scaffolding: tight sites, terraces, shared access or the need for road permits make the job slower and more expensive.
- Electrics, heating and insulation: extending the heating, adding sockets and lighting, and meeting thermal requirements all add up. Any new circuits must comply with Part P, so the work must be done by a registered electrician or notified to building control.
- Professional fees: architect or designer drawings, structural engineer calculations, building control fees and any party wall agreements are separate from the build itself.
- Location and demand: labour costs vary by area, and good loft firms are often booked months ahead.
Ways to keep the cost down
- Check whether a Velux conversion will do. If the headroom is good, you may not need a dormer.
- Keep the layout simple. Put the stairs above the existing stairwell where you can, and keep any bathroom close to existing plumbing stacks.
- Choose standard windows and fittings. Bespoke items cost more and take longer to arrive.
- Decide everything before work starts. Changes mid-build are the most common cause of extra charges.
- Clear the loft yourself and prepare access, so you are not paying for labour on tasks you could do.
- Get several detailed quotes that are written on the same basis, so you can compare like with like.
- Handle some finishing yourself if you are competent, such as painting and flooring, but leave structural, electrical and fire-related work to professionals.
- Do not cut corners on design and structure. Mistakes found later cost far more to fix than getting the drawings right at the start.
What should be included in a quote
A proper quote is itemised and clear about what is and is not covered. Look for:
- [ ] Full description of the conversion type, size and layout, with reference to drawings
- [ ] Who is responsible for structural calculations, building regulations and planning checks
- [ ] Building control fees and who pays them
- [ ] Party wall notices and surveyor costs, if relevant
- [ ] Scaffolding, including how long it is hired for
- [ ] Steelwork and floor strengthening
- [ ] Roof work, windows, flashing and weatherproofing
- [ ] Insulation and ventilation specification
- [ ] Staircase supply and fitting, including balustrade
- [ ] Electrics, lighting, sockets, smoke and heat alarms
- [ ] Plumbing and heating, including radiators and any bathroom fit-out
- [ ] Fire doors and any fire protection to the floors below
- [ ] Plastering, joinery, flooring and decorating, or a clear note that they are excluded
- [ ] Waste removal and site protection for the rest of the house
- [ ] Payment schedule tied to stages of work
- [ ] Start date, estimated duration and how variations will be priced
- [ ] Insurance details and guarantees
Questions to ask a builder
- Have you completed loft conversions like mine, and can I see or speak to past clients?
- Who deals with building control, and will I receive the completion certificate?
- Is the structural engineer your own or independent, and are their calculations included?
- Do you carry public liability insurance, and what warranty or guarantee do you offer?
- Are electricians and gas or heating engineers properly registered for their work?
- What is not included in this price?
- How will you protect the rest of the house from dust and water?
- What happens if you find rot, poor timbers or other problems once the work is open?
- Who will be on site day to day, and will the same team stay throughout?
- How and when do I pay, and how are changes agreed in writing?
How long it takes and what to expect
A Velux conversion usually takes less time than a dormer, but both run for several weeks at least, and larger or more complex jobs can take a couple of months or more. The design and approvals stage before building starts can take as long as the build itself.
Expect scaffolding early on, followed by noise and dust while the roof is opened up. A good firm weatherproofs the roof quickly, but you should still expect some disruption, with workers in and out of the house, and the stairs area affected. Utilities may be switched off briefly for electrical and plumbing work.
Building control will visit at stages, such as after structural work and before insulation is covered. Do not pay the final instalment until the work is finished to your satisfaction and you have the sign-off certificate. Snagging is normal, so walk through the room in good light and list anything that needs correcting.
Signs a quote is too cheap or too expensive
Too cheap:
- It is vague, with no breakdown or drawings.
- It leaves out structural calculations, building control or scaffolding.
- The builder is happy to skip building regulations.
- They ask for a large payment upfront.
- They cannot show insurance or examples of earlier work.
- It is far lower than the others without a clear reason. Missing items usually reappear later as extras.
Too expensive:
- Large allowances are listed as lump sums with no explanation.
- Specification is higher than you asked for.
- The builder is unwilling to explain how the figure is made up.
- It includes work you did not request.
- It is much higher than comparable quotes for the same scope.
Compare the prices on this page with your quote. Then ask each builder to explain any big gaps line by line. A fair quote is clear about what you get, who does it and how changes are handled.