Loft Conversion Types Compared: Dormer, Hip-to-Gable, Velux, Mansard

"Loft conversion" covers several genuinely different types of project, and picking the wrong one for your roof shape is one of the most common early mistakes — some types simply aren't possible on some houses, and the cost difference between them is significant.

Roof light (Velux) conversion

Works within the existing roof shape — no structural alteration to the roofline, just roof windows, insulation, flooring and a staircase. The cheapest and least disruptive type, but it only works where the existing roof pitch already gives enough headroom to stand comfortably across most of the space.

• Typical cost: £30,000–£40,000
• Typical timeline: 4–6 weeks
• Best suited to: roofs with generous existing pitch and headroom; anyone prioritising cost and minimal disruption over maximum space.
• Watch out for: headroom that looks sufficient by eye but doesn't actually meet building regulations minimums once measured properly.

Dormer conversion

Adds a box-shaped extension to the roof slope, creating genuine additional headroom and floor space rather than working within the existing pitch. The most common loft conversion type in the UK, suited to a wide range of roof shapes, and a reasonable middle ground on cost and disruption.

• Typical cost: £40,000–£55,000
• Typical timeline: 6–8 weeks
• Best suited to: most pitched-roof houses; a good balance of extra space against cost and build time.
• Watch out for: dormer size and position rules under permitted development — see our permitted development guide.

Hip-to-gable conversion

Extends a hipped (sloped) end of the roof out to a vertical gable wall, which significantly increases usable space at that end of the loft. Common on detached and semi-detached houses with a hipped roof end — terraced houses with a gable end already don't need this.

• Typical cost: towards the higher end of the £40,000–£75,000 range, depending on scale and whether it's combined with a rear dormer.
• Typical timeline: 7–9 weeks
• Best suited to: detached and semi-detached houses with a hipped roof end, often combined with a dormer for maximum usable space.
• Watch out for: this is only relevant if your roof actually has a hipped end — a terraced house with a gable end skips this option entirely.

Mansard conversion

The most involved type — replaces most of one roof slope with a near-vertical wall at a steep angle, maximising usable space and headroom across the whole loft. The most structurally significant change, and typically the most expensive of the four.

• Typical cost: £55,000–£75,000+
• Typical timeline: 8–12 weeks
• Best suited to: anyone prioritising maximum usable space and headroom over cost and build time, and properties where a full roof rebuild is otherwise justified.
• Watch out for: mansard conversions more often need full planning permission rather than permitted development, given the scale of the change to the roof's appearance.

Comparing them side by side

• Least structural change, lowest cost: Velux
• Balanced space gain vs. cost: Dormer
• Best for hipped-roof houses specifically: Hip-to-gable
• Maximum space, highest cost: Mansard
• Most likely to fall under permitted development: Velux and dormer, subject to the usual size/position conditions
• Most likely to need full planning permission: Mansard, and hip-to-gable in some conservation areas

See our full loft conversion cost guide for more detail on price ranges by type, and our labour costs guide for why the more structural types cost more to build, not just to design.

A simple decision checklist

• Does your existing roof pitch already give enough headroom? If yes, a Velux conversion may be all you need.
• Do you need significantly more headroom or floor space than the existing pitch allows? A dormer is the most common answer.
• Does your roof have a hipped end? Hip-to-gable becomes a realistic option, often paired with a dormer.
• Is maximum space the priority regardless of cost and build time? A mansard conversion delivers the most, at the highest cost and longest build.

Which one is right for you?

The honest answer depends on your roof shape, your property type, how much space you actually need, and your budget — a Velux conversion for a home office or single extra bedroom is a very different proposition to a mansard conversion aiming to add a full principal bedroom with en-suite. A proper site assessment is the only reliable way to know which types are even structurally possible before comparing cost between them. See how long each type typically takes for the timeline side of the same decision.