Loft Conversion: Complete Guide to Types, Costs & Planning 2026
A loft conversion is one of the most cost-effective ways to add a bedroom, bathroom, or home office without losing garden space. In Dorset, where property prices make moving expensive, converting your loft delivers genuine value.
What you need to know
Types of loft conversion
There are four main types: (1) Velux/rooflight — the simplest, adding windows to the existing roof slope without changing the roof shape; (2) Dormer — extends vertically from the roof slope, creating more headroom and floor area; (3) Hip-to-gable — extends the sloped (hipped) side of the roof to a vertical gable wall; (4) Mansard — replaces one roof slope with an almost-vertical wall and flat roof, creating maximum space.
Which type suits your home?
Velux conversions suit homes with steep roof pitches and adequate ridge height. Dormers work on most house types. Hip-to-gable is specific to hipped-roof properties (many 1930s semis). Mansard conversions are the most complex but create the most space — popular on terraces and in conservation areas where other options are limited.
Structural requirements
All loft conversions require: a floor structure upgrade (existing ceiling joists are not strong enough to be floor joists), a new staircase, fire protection (fire doors, escape windows, or protected route), and insulation to current standards. The existing roof structure may need steel beams to open up the space.
The staircase question
The staircase is often the biggest design challenge. It needs to be positioned above the existing staircase where possible, or it will eat into a room below. Building Regulations set minimum headroom and pitch requirements. We design the staircase position first, then build the layout around it.
How much does it cost?
| Type | Typical Cost Range |
|---|---|
| Velux/rooflight conversion | £30,000–£45,000 |
| Single dormer | £40,000–£60,000 |
| Double dormer (L-shaped) | £50,000–£70,000 |
| Hip-to-gable + dormer | £55,000–£75,000 |
| Mansard conversion | £65,000–£90,000 |
| En-suite bathroom addition | £5,000–£10,000 |
| Professional fees | £2,000–£5,000 |
| Building control fees | £400–£800 |
Prices based on Dorset rates, 2026
Planning permission
Velux and rear dormer conversions usually fall under Permitted Development. Front-facing dormers, hip-to-gable conversions, and mansard conversions typically require planning permission. In conservation areas and National Parks, most loft conversions need planning.
Under PD, loft conversions can add up to 40m³ of space (50m³ for detached houses). The materials must match the existing house. No dormer can extend beyond the plane of the existing roof slope facing a highway. The conversion must not exceed the existing ridge height.
Building regulations
Building Regulations for loft conversions are extensive: structural floor upgrade, fire safety (30-minute fire doors, protected escape route or escape windows), thermal insulation (0.16 W/m²K for roofs), sound insulation between the conversion and rooms below, adequate headroom (2.2m at the centre), and electrical certification.
Typical timeline
| Phase | Duration |
|---|---|
| Design | 2–3 weeks |
| Planning (if required) | 0–8 weeks |
| Structural calcs and Building Regs | 2 weeks |
| Scaffolding and roof work | 2–3 weeks |
| Internal fit-out | 4–6 weeks |
| Staircase installation | 1 week |
| Finishing and snagging | 2 weeks |
Frequently asked questions
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