Key facts
- Typical cost
- £30,000 to £45,000 rooflight; £45,000 to £65,000 dormer; £55,000 to £80,000 hip-to-gable or mansard
- Typical timescale
- 6 to 10 weeks on site
- Minimum head height
- Around 2.2 m measured from the top of the ceiling joists to the underside of the ridge is a useful starting point
- Permissions
- Building Regulations always; many dormers fall under permitted development
Figures are indicative for Northamptonshire at the time of writing and exclude VAT where applicable. Your quote is fixed and specific to your project.
What's included
- Feasibility check of head height and structure
- Structural calculations, steels and new floor joists
- Dormer, hip-to-gable or rooflight construction
- New staircase designed for head height and fire escape
- Fire doors and interlinked mains smoke alarms
- Insulation to current Building Regulations
- En-suite plumbing and heating extension
- Electrics, plastering and decoration
Is my loft suitable for conversion?
Measure from the top of a ceiling joist to the underside of the ridge timber. Around 2.2 metres or more usually means a straightforward conversion; less than that may need a dormer, a hip-to-gable or lowering ceilings below. Traditional cut roofs with rafters and purlins are easier to convert than modern trussed roofs, which need more steelwork. We can confirm suitability on a single visit.
Which type of loft conversion is right for my house?
A rooflight conversion keeps the roof shape and adds windows in the slope, which is the most affordable option when there is already enough head height. A rear dormer adds a box structure with vertical walls and is the most popular choice for semis and terraces. A hip-to-gable replaces a sloping side roof with a vertical gable, often combined with a rear dormer on semis and detached houses. A mansard rebuilds the rear slope almost vertically and usually needs planning permission.
Do loft conversions need planning permission?
Many do not. Permitted development allows up to 40 cubic metres of additional roof space for terraced houses and 50 cubic metres for semi-detached and detached houses, provided nothing extends beyond the front roof slope facing the road, nothing is higher than the existing ridge, and side windows are obscure glazed. Conservation areas and flats are excluded. Building Regulations approval is always needed, covering structure, fire safety, stairs and insulation. Read our loft conversion guide for more.
Loft conversions FAQs
How long does a loft conversion take?
Most loft conversions take 6 to 10 weeks on site. Rooflight conversions are quickest; hip-to-gable and mansard conversions take longest because more of the roof is rebuilt.
Can I stay at home during a loft conversion?
Yes. Most of the work is done from scaffolding outside, and the house below is only opened up near the end, when the new staircase goes in.
Do I need fire doors for a loft conversion?
In a typical two-storey house becoming three storeys, Building Regulations require a protected escape route to the front door. That usually means fire doors to habitable rooms off the stairs and mains-powered, interlinked smoke alarms on every level.
Can you add an en-suite in the loft?
Yes. Our plumbers plan the soil pipe route and water supply at design stage, and we check your boiler or cylinder can cope with the extra bathroom.
Does a loft conversion add value?
Adding a bedroom and bathroom usually adds more value than it costs in most of Northamptonshire, but it depends on your street's ceiling price. An estate agent can advise on your specific house.