How much does a mansard roof extension cost?
Mansard roof extensions are one of the most effective ways to gain a full extra storey – especially on period homes in London and other historic cities. Below, we share UK costs from our architects’ survey, with a clear breakdown for London & South East vs outside London, plus why a mansard extension typically costs more than a dormer.
Related reading: How much does a roof extension cost?
Image above: A homeowner found their architect, Justin on Design for Me for their mansard roof extension project. Here’s a link to the case study to find out more about the design process. See Justin’s full profile and shortlist him for your mansard roof extension project.
1) Mansard roof extension cost (UK average)
From our survey, the average costs were:
- Construction cost: £107,425
- Architects’ fees: around £11,800 (about 11% of construction – see our architects’ fee guide)
- Other consultants: £5,000
- Total project cost (UK average): ~£124,200
2) Mansard roof extension cost in London & South East
Costs are typically higher in London and the South East:
- Construction cost: £109,804
- Architects’ fees: around £14,300 (about 13% of construction)
- Other consultants: £6,421
- Total project cost (London & South East): ~£130,500
3) Mansard roof extension cost outside London & South East
Outside London and the South East, costs are lower on average:
- Construction cost: £97,385
- Architects’ fees: around £8,800 (about 9% of construction)
- Other consultants: £3,466
- Total project cost (outside London & South East): ~£109,600
Why are mansard roof extensions more expensive?
A mansard roof creates almost a full extra storey, so the build is more involved than adapting existing loft space. Expect:
- More structure – new walls, steelwork, roofing and weathering.
- Facade considerations – a rear mansard roof extension on a period property often requires sensitive design, especially in conservation areas.
- Access & logistics – scaffolding, weather protection, and tight city sites all add cost.
- Professional input – planning strategy, structural design and party wall matters often require multiple consultants.
Despite the higher upfront cost, a modern mansard roof or contemporary mansard roof can add significant long-term value and usable space.
Dormer vs mansard: cost comparison
From our survey, the average construction cost of a typical dormer loft conversion was £88,000, compared with £109,804 for a mansard roof extension (London & South East). If you only need a single extra room and don’t require full head height, a dormer can be more economical. If you want a full extra storey, a mansard dormer or full mansard is often the better investment.
Get inspired: Loft extension ideas | Planning for a mansard roof extension
FAQs
What is a mansard roof extension?
A mansard extension replaces the rear (and sometimes front) roof pitch with a near-vertical face and a flatter top, creating a generous new storey with better head height than a dormer.
Do I need planning permission for a mansard roof?
Often yes — particularly in conservation areas or on period terraces. Policies vary by borough, so early planning advice is important.
How long does a mansard extension take to build?
Many projects take a few months once permissions and party wall matters are resolved, though timings depend on access and scope.
Can a mansard be modern on a period home?
Yes. A contemporary mansard roof might use slate, zinc or standing-seam metal for a crisp, modern look. Design and context will guide the right material choice.
More cost planning: Roof extension costs · Architects’ fees for extensions
