Dormer vs Mansard Loft Conversion: Which Should You Choose?
Both dormers and mansards add headroom to your loft. But they are very different in cost, complexity, and planning requirements. A dormer modifies part of the roof; a mansard replaces an entire roof slope. This guide compares them.
The details
What is each type?
A dormer is a boxed projection from the existing roof slope — it adds headroom and floor space to a section of the loft. A mansard replaces an entire roof slope with a near-vertical wall topped by a flat or shallow-pitched roof. Mansards create a full-height room across the entire width of the house.
Planning requirements
Rear dormers usually fall under PD (within volume limits). Mansard conversions almost always need planning permission because they fundamentally change the roof shape and add significant volume. In conservation areas, both may need planning, but mansards face stricter scrutiny due to their greater visual impact.
Cost and headroom
A rear dormer typically costs £48,000–£72,000 and adds 10–15m² of full-height space. A mansard costs £65,000–£100,000+ and adds 25–40m² of full-height space. Per m² of usable headroom, the mansard is better value — but the upfront cost is significantly higher.
| Factor | Dormer | Mansard |
|---|---|---|
| Cost | £48,000–£72,000 | £65,000–£100,000+ |
| Full-height space | 10–15m² | 25–40m² |
| Planning needed? | Usually PD (rear) | Almost always |
| Build time | 8–12 weeks | 12–18 weeks |
| Structural impact | Moderate | Significant (new roof structure) |
| Best for | Adding a bedroom + bathroom | Full additional floor |
| Property types | Most houses | Terraced, semi-detached |
Our verdict
Choose a dormer if you want to add a bedroom and bathroom without the cost and planning complexity of a mansard. Choose a mansard if you want to maximise loft space across the full width of the house and are willing to go through the planning process.
Common questions
Frequently asked questions
Ready to take the next step?
Fixed-price plans drawn by qualified drafters. No obligation, no follow-up calls.
Every plan drawn by qualified drafters in Poole, Dorset
Serving homeowners across Poole, Bournemouth, Christchurch, Wimborne, Wareham, and the wider Dorset area.