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Build a mansard roof.
Steep-pitched mansard roof — converts loft to full habitable storey. Mainly for terraced houses.
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Planning rejection in conservation areas
Mansards must match neighbouring properties' rooflines. Get pre-application advice.
Party wall disputes
Mansards usually involve raising shared walls. Party Wall Award required.
Inadequate ventilation in steep face
Modern mansards need careful Part F ventilation detail.
Stair compromise
Adding an extra floor needs Building Regs Part K compliant stair. Often eats existing room space below.
Costs & timeline
Know what it costs. Know when it ends.
Indicative UK ranges and a typical week-by-week schedule.
Cost range
By job type
Inc. VAT · 2026
Source: NMT quotes
Source: NMT quotes
Mansard conversion (small terrace)30–40 m² new floor
£25k–£55k
Mansard conversion (semi-detached)40–60 m² new floor
£35k–£80k
Mansard conversion (detached)60+ m²
£50k–£120k
Slate steep-face cladding per m²Welsh slate
£120–£250/m²
Tile cladding per m²Concrete or clay
£60–£120/m²
Lead roll detail per metreHeritage finishing
£120–£250
!
Quote spread is typically ± 18% — always get 3 quotes.
Timeline
Semi-detached full re-roof · week by week
Typical
4 phases · 3 wk
4 phases · 3 wk
W1
W2
W3
Scaffold erection
Strip + felt + battens
Tile + ridge + verge
Flashings + snags
!
Schedule slips on dependencies — pad each phase by 10–20% for real-world delays.
At a glance
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Roofer explained
Mansard roofs have two pitches per side — a steep lower section (60–70°) and a shallow upper section (15–30°). Originally French (Mansart, 17th century), they maximise loft space within a small footprint. Expect to pay £30,000–£90,000 for UK mansard roof construction or conversion.
What Does the Work Involve?
Mansards are typically built as part of a loft conversion or new build. Existing roof stripped, eaves walls raised, mansard frame constructed. Steep lower face usually clad in slate or tile (visible from street); upper pitch typically tile. Dormer windows pierce the steep face. Internal lining, insulation, electrical and plumbing for new room.
Typical Costs
| Item | Low (£) | High (£) | Notes |
|---|---|---|---|
| Mansard conversion (small terrace) | 25,000 | 55,000 | 30–40 m² new floor |
| Mansard conversion (semi-detached) | 35,000 | 80,000 | 40–60 m² new floor |
| Mansard conversion (detached) | 50,000 | 120,000 | 60+ m² |
| Slate steep-face cladding per m² | 120 | 250 | Welsh slate |
| Tile cladding per m² | 60 | 120 | Concrete or clay |
| Lead roll detail per metre | 120 | 250 | Heritage finishing |
| Architect/structural design | 2,500 | 8,000 | 10% of build cost |
How Long Does It Take?
Mansard loft conversion: 10–16 weeks total. Roof framing 2–4 weeks; cladding 2–3 weeks; internal fit 4–6 weeks; finishes 2–3 weeks.
DIY or Professional?
Professional only. Architect/structural engineer for design. Specialist loft conversion company or builder for execution.
Choosing the Right Tradesperson
Specialist mansard loft conversion company. Get 3 quotes with detailed spec. Confirm architectural design, structural calcs, and party-wall agreements (if shared walls) are included.
UK Regulations
Mansards often need planning permission — many councils have specific mansard policies in conservation areas. Building Regs notification mandatory (structure, Part L, Part B). Party Wall Act for shared walls.
Common Problems
- Planning rejection in conservation areas: Mansards must match neighbouring properties' rooflines. Get pre-application advice.
- Party wall disputes: Mansards usually involve raising shared walls. Party Wall Award required.
- Inadequate ventilation in steep face: Modern mansards need careful Part F ventilation detail.
- Stair compromise: Adding an extra floor needs Building Regs Part K compliant stair. Often eats existing room space below.
Frequently Asked Questions
Why choose a mansard?
Maximum loft space within smallest visible roof change. Provides full headroom and floor area — often equivalent to a full extra storey.
Do I need planning permission?
Almost always — mansards rarely fall under permitted development. Conservation areas have specific policies; mansards are scrutinised carefully.
How much value does a mansard add?
Typical addition: 15–25% to property value. Cost recovery 70–90% of build cost on a quality conversion.
Mansard vs dormer conversion?
Mansard: larger floor area, more roofline change, more cost (£30K–£90K). Dormer: smaller, less visible change, less cost (£7.5K–£35K).
Can I convert any roof to a mansard?
Most pitched roofs can be converted. Steep existing roofs (already 50°+) may not benefit. Get architect assessment first.
How long does a mansard last?
Structure: 100+ years. Roof finish: 60+ years (concrete tile), 100+ years (slate). Comparable to standard pitched roof.
This guide was written with AI assistance and is intended for general information only. Prices are estimates based on UK averages and may vary by region. Always get at least three quotes and consult a qualified professional before starting any work.
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