What is the mansard wall pitch rule?

Typically 70–72°, with flat or shallow-pitch roof above. Borough-specific.

Can I combine mansard and rear extension?

Yes, but phasing usually wins — PD rear extension first, then full-planning mansard 1–2 years later.

Costs · Updated July 2026

How Much Does a Mansard Loft Conversion Cost in 2026? (UK)

Q: Why is a mansard so much more expensive than a dormer?

Mansard replaces the full roof slope, not a section. Bigger steels, planning required, party-wall work — typically 60–90% more than a dormer.

Q: Do mansards always need planning permission?

Almost always yes. Roof shape change disqualifies most from PD.

Q: How much value does a mansard add?

£100k–£180k in London zones 2–6 with ensuite. £60k–£100k outside London.

Q: How long does a mansard take?

14–22 weeks on site, 14–20 weeks pre-construction. 6–10 months total.

A mansard loft conversion in 2026 UK costs £55,000–£95,000 fully finished — the most expensive loft type, but it gives by far the most usable space (typically 35–55m² vs 18–28m² for a Velux). Mansard works by replacing the entire roof slope with a near-vertical (72°+) wall and a flat roof above — almost always needing full planning, party-wall agreements, and structural redesign. Best fit: Victorian/Edwardian terraces in London zones 2–6 where the typical mansard adds £100,000–£180,000 of value. This guide walks the 2026 cost ladder, the 6 cost drivers, and the planning/PD reality.

£55k–£95k typical Biggest space gain Planning required

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Cost Drivers

The 6 Things That Drive Mansard Loft Conversion Cost

1. Floor area & ceiling height

Mansard delivers full-height standing across nearly the whole floor — typically 35–55m². Bigger footprint = bigger cost, but cost-per-m² actually drops as size grows.

2. Planning permission

Mansards almost always need full planning. Pre-application advice £300–£900, full app £258, design-and-access statement £400–£900. Allow 16–22 weeks total.

3. Party-wall friction

Terraced mansards trigger Section 1 + 2 + 6 notices. Each neighbour: £1,200–£2,400 with appointed surveyors. Hostile neighbours can drag this to £5,000+.

4. Structural rebuild

Major steels, party-wall remedials, brick mansard wall to match. Structural engineer £1,200–£2,500. The full roof replacement is the biggest single line item.

5. Windows: sashes or Velux?

Conservation areas usually mandate vertical sashes (£900–£1,500 each) on the mansard face. Velux can be added on the flat roof above for daylight.

6. Internal layout: ensuite or two rooms?

Bedroom + ensuite: £8,000–£14,000 extra. Two bedrooms + shared bathroom: £12,000–£20,000 extra. The biggest single resale move you can make at the top of a terrace.

Visual guide

Mansard costs compared

Where this job sits against the other loft conversions jobs, on 2026 UK prices. Your own quote depends on size, access and spec.

Source: BestBuilders 2026 pricing across 10 loft conversions jobs. The highlighted bar is this page.

Frequently Asked

Common Questions

Why is a mansard so much more expensive than a dormer?+

Mansard replaces the whole roof slope, not just a section. Bigger steels, bigger party-wall work, planning permission required and brick wall to match the existing house — typically 60–90% more than a dormer.

Do mansards always need planning permission?+

Almost always yes. The roof shape change disqualifies most mansards from PD. Some London boroughs have explicit mansard PD policies but conservation areas always need full planning.

How much value does a mansard add?+

£100,000–£180,000 typical uplift in London zones 2–6 if the conversion adds a bedroom with ensuite. Outside London, uplift is £60,000–£100,000.

How long does a mansard take?+

14–22 weeks on site, plus 14–20 weeks of design, planning and party-wall lead time. Total: 6–10 months from kick-off to handover.

What is the planning rule on mansard wall pitch?+

Typically the mansard wall must be at 70–72° (not vertical), with a flat or shallow-pitch roof above. Many London councils have specific design guides — read them before drawing.

Can I do a mansard and a rear extension together?+

Yes, but combined planning is harder. Most successful projects separate them: rear extension first under PD, then mansard 12–24 months later with full planning.

Keep Reading

Related Guides

More guides in Costs.

Go deeper

Mansard — guides and detail

The detail behind the number: what changes the price, the planning position, and how to check a quote before you accept it.

Mansard planningWhy a mansard always needs consent.See costs & get quotes →Is a mansard worth it?Most space, most cost, most planning risk.See costs & get quotes →Mansard loft conversion calculatorThe most space and the most cost — planning is always needed.See costs & get quotes →

Reviewed by the BestBuilders editorial team on 25 July 2026 · Next scheduled review: October 2026 · See our editorial standards.