Mansard Loft Conversion Cost UK (2026): Prices and Planning

How much does a mansard loft conversion cost?

A mansard replaces the existing roof slope – usually at the back, sometimes front and back – with a near-vertical wall (around 70°) topped by a flat or very shallow roof. The party walls are built up on each side, and windows sit in small dormers within the steep slope. The result is effectively a new top storey with full headroom across almost the whole floor, which is why mansards are so common on Victorian and Georgian terraces in London and other cities.

That extra space comes at a price. Published 2026 figures:

• MyJobQuote (September 2026): average £67,500, typical range £55,000–£80,000, and £70,000–£90,000 in central London.
• MyBuilder: £50,000–£80,000, with London £70,000+ and northern regions around £50,000 for a similar scope.
• HomeOwners Alliance (July 2026, Checkatrade data): average about £65,000.
• PlanWatch (October 2026): £65,000–£100,000+, or £85,000–£120,000+ in London.
• Simply Loft, a London loft company, told Homebuilding & Renovating that mansards “typically cost £70,000–£80,000 + VAT or more”.

Our calculator figures sit at the upper end of these ranges because they include design fees, a structural engineer, building control, the £548 planning fee and VAT – costs that headline builders’ prices often leave out.

Mansard cost by size

Usable floor area	Typical layout	Estimated total cost (standard finish)
25m²	Double bedroom (small terrace)	£52,500 – £79,000
30m²	Double bedroom with shower room	£62,500 – £93,500
35m²	Bedroom suite or bedroom plus study	£72,500 – £108,500
40m²	Two bedrooms and a bathroom	£82,500 – £123,500
50m²	Front and rear mansard on a wide house	£103,000 – £153,000

MyJobQuote’s size table is lower – £30,000–£45,000 for 30m², £40,000–£60,000 for 40m² and £50,000–£75,000 for 50m² – because it reflects the core build only. Treat those as the floor of the range, not the final bill.

Mansard cost per m² and by finish

Finish level	Build cost per m² (ex VAT)	35m² all-in (Midlands)
Basic – felt or EPDM flat roof, standard windows	£1,480 – £2,090	£65,000 – £97,000
Standard – natural slate slope, mid-range shower room	£1,650 – £2,350	£72,500 – £108,500
High-end – full-width glazing, quality en-suite, bespoke joinery	£2,090 – £2,870	£91,500 – £131,500

Per-m² rates are higher than for a dormer because a mansard rebuilds a whole roof slope, raises the party walls and usually needs more steel. Materials matter too: MyBuilder notes that premium slate or tiles can add 20–30% to the roofing cost compared with basic options.

Mansard costs by region – and in London

Region	Build cost per m² (ex VAT)	35m² mansard, all-in
London	£2,062 – £2,938	£92,000 – £137,000
South East	£1,848 – £2,632	£82,500 – £123,500
South West & East of England	£1,716 – £2,444	£77,000 – £115,000
Midlands	£1,617 – £2,303	£72,500 – £108,500
North of England & Wales	£1,518 – £2,162	£68,000 – £102,000
Scotland & Northern Ireland	£1,584 – £2,256	£71,000 – £106,500

Mansard roof extension cost in London

London is where most UK mansards are built. MyJobQuote’s October 2026 London data puts a 30m² mansard at £70,000–£90,000 and a 50m² mansard at £85,000–£105,000, with an average around £80,000. SMA Lofts, a London loft company, quotes £60,000–£85,000 including VAT on its published price page. Our calculator gives £79,000–£118,000 for a 30m² London mansard once all fees, planning and VAT are added.

London mansards cost more because of higher labour rates, restricted access on terraced streets, council licences for scaffolding and skips, and the near-certainty of party wall agreements on both sides. Our London loft conversion cost guide goes into borough planning and conservation areas in more depth.

Mansard cost breakdown

Cost item	Typical cost	Notes
Structure (roof removal, party wall build-up, steels, new frame)	£15,000 – £30,000	MyBuilder component estimate
Scaffolding	£4,500 – £7,500	Often a full roof scaffold with a temporary roof (MyJobQuote)
Steel beams	£800 – £2,500 each	MyJobQuote
Staircase installation	£1,800 – £2,500	MyBuilder; bespoke stairs cost more
Design and drawings	4–7% of build cost	Planning drawings plus building regulations drawings
Structural engineer	£500 – £2,500	Essential for a mansard
Building control	£600 – £1,200	Always required
Planning application	£548	England householder fee from 1 April 2026
Party wall surveyors	£1,000 – £3,000 per neighbour	Mid-terrace mansards usually affect two neighbours
Bat survey (if requested)	£300 – £1,000	Sometimes required when an old roof is removed
VAT and contingency	20% VAT; 10% contingency	VAT not charged on the planning fee

MyJobQuote estimates labour at 40–50% of the total – about £27,000–£34,000 on an average project – with carpentry and structural work alone at £14,000–£18,000.

Mansard vs dormer: which is better value?

Comparison	Rear dormer	Rear mansard
30m² all-in cost (Midlands, standard)	£46,500 – £70,500	£62,500 – £93,500
Average cost (HomeOwners Alliance)	£50,000	£65,000
Planning permission	Usually permitted development	Almost always needed
Headroom and usable space	Good, but sloping front ceiling remains	Near full headroom across the floor
Typical build time	6–10 weeks	8–16 weeks
Appearance	Box-like; can look bulky	Traditional; often preferred in conservation areas

A dormer is cheaper and faster, and usually avoids a planning application. A mansard costs roughly a third more but gives a more generous, regular room and is often the only roof extension a council will approve in a conservation area, where box dormers are typically refused. If you are on a street where neighbours already have mansards, the case for one is strong. See our dormer loft conversion cost guide to compare in detail.

Mansard planning permission

Because a mansard raises the party walls and changes the roof profile, it almost never fits within loft permitted development rules. Budget for a householder planning application (£548 in England) and around eight weeks for a decision – longer in conservation areas.

The good news is that national policy is supportive. Since December 2023 the National Planning Policy Framework has told councils to allow mansard roof extensions on suitable properties where the external appearance harmonises with the original building. Many London boroughs also publish mansard design guidance, and some go further: the Royal Borough of Kensington and Chelsea adopted a Local Development Order in April 2024 allowing mansards at 21–32 Redcliffe Road without a planning application, provided they follow its design brief.

Applications are most likely to succeed when they:

• follow an established pattern of mansards on the same terrace;
• keep the front slope at a traditional angle (around 70°) with a parapet or eaves line preserved;
• use natural slate and timber-framed dormer windows in conservation areas;
• avoid overlooking with rear terraces or balconies, which are often refused.

Listed buildings also need listed building consent. Building regulations approval is always required, covering structure, fire escape, insulation and stairs.

How long does a mansard take?

1. Design and planning drawings: 3–6 weeks.
2. Planning decision: about 8 weeks, often 8–13 weeks in practice (MyJobQuote).
3. Party wall notices: served at least two months before work; run them alongside planning.
4. Build: estimates vary – MyJobQuote gives 6–12 weeks depending on complexity, while MyBuilder gives 12–20 weeks including planning and delays. Allow 10–14 weeks on site for a typical rear mansard.
5. Sign-off: 1–2 weeks for snagging and completion certificate.

How to save money on a mansard

• Copy an approved neighbour’s design – it speeds up planning and reduces design fees.
• Choose a rear-only mansard rather than front and rear, unless the street pattern demands both.
• Agree party wall matters early, ideally with one agreed surveyor acting for both sides.
• Use man-made slate where the council will accept it.
• Get three itemised quotes and check whether scaffolding, temporary roofing and party wall work are included.

Does a mansard add value?

Simply Loft’s managing director told Homebuilding & Renovating that a well-executed mansard can add up to 20% to a home’s value, especially in high-demand areas. Nationwide research cited by Which? found a loft conversion adding a bedroom and bathroom can add up to 24%. In prime London streets, where buyers pay heavily per square foot, a mansard’s extra space often makes it the best-value loft option despite the higher cost.

Mansard loft conversion FAQs

How much does a mansard loft conversion cost?

Most mansards cost £60,000–£110,000 all-in, with averages of around £65,000–£67,500 for the build. Our calculator puts a 35m² standard-finish mansard at £72,500–£108,500 including planning, fees and VAT in the Midlands.

How much does a mansard cost in London?

MyJobQuote puts a 30m² London mansard at £70,000–£90,000 and 50m² at £85,000–£105,000. Including all fees, planning and VAT, our calculator gives about £79,000–£118,000 for 30m².

Do mansard loft conversions need planning permission?

Almost always. A mansard changes the roof profile and raises party walls, which takes it outside permitted development. National policy is supportive where the design suits the building and street.

Is a mansard more expensive than a dormer?

Yes, typically by about a third. HomeOwners Alliance averages are £65,000 for a mansard and £50,000 for a dormer, and a mansard also needs a planning application.

Can you build a mansard in a conservation area?

Often, yes – a well-designed mansard is frequently the preferred roof extension in conservation areas, particularly where neighbours already have one. Expect the council to require traditional materials such as natural slate.

How long does a mansard loft conversion take?

Allow around 4–6 months from first drawings to completion: 2–3 months for design and planning, then roughly 10–14 weeks on site.

Related guides: Loft conversion cost calculator · Party wall agreements · Party wall surveyor cost

---