When to explore this option
Explore this where a substantial roof redesign is under consideration. Begin with the external design and consent route, then develop the internal layout. Avoid assuming that a large internal floor area means the proposal is acceptable in its setting.
Mansard or dormer?
A dormer projects from an existing roof slope. A mansard changes the overall roof profile, with a steep lower face and a shallower upper part. Ask for external elevations of both approaches and an internal layout drawn to the same scale. Compare the usable room alongside the effect on the house and neighbouring roofline.
Begin with the planning route
A mansard is a substantial roof alteration and typically needs planning permission. Check that route with your designer before commissioning a detailed construction quote. A neighbouring mansard can provide context, but it does not confirm that a similar scheme will be acceptable on your home.
Our planning guide explains why planning and building regulations are separate checks.
Compare design costs as well as building work
Ask what is included for surveys, drawings, structural design, applications and revisions. Then compare construction against the same design and finish. Clarify the allowance for weather protection, roof coverings, windows, stairs and work to adjoining parts of the house. Use the cost guide before comparing headline totals.
Questions for your first assessment
- How does the proposed roof relate to adjoining buildings?
- What planning advice is needed before detailed pricing?
- How will the roof structure be built and weather-protected?
- Are design changes during the planning process included in the fee?
Get clarity before comparing prices
Ask for a clearly described design stage and construction stage. An early outline allowance may be useful for deciding whether to continue, but it is not a fixed quote for a final approved scheme. Record the assumptions attached to every price.
What to include in your enquiry
Include your postcode, property type, intended room, budget and timing. Mention any drawings and rooms that need to stay usable during the work. You do not need to choose a conversion type before enquiring.
Permissions and technical checks
Read our planning guide and building regulations guide before discussing the assessment. They cover different checks; a builder introduction does not establish that either approval is in place.
Sources & review
Published by Coalesce Local Ltd. Reviewed 19 September 2026.