Loft conversion cost in 2026 — full UK price guide
A loft conversion is the highest-ROI project most UK homeowners ever run — typical resale uplift is 15–20% on a three-bed property, and a well-designed loft pays for itself within 6–8 years. This guide breaks down 2026 costs for every loft-conversion type, the regional variation across London and the South East, and the design and planning factors that move the figure up or down. All numbers are real RCB Design & Build delivery costs from the last 18 months, not industry-averaged guesses.
Loft conversion cost by type (London/M25 corridor, 2026).
Project type
From
To
Notes
Velux-only (rooflight) conversion
£35,000
£55,000
No structural roof change. Quickest type — 6–8 weeks on site.
Rear dormer conversion
£55,000
£75,000
Most popular London type. Single rear dormer adds full headroom.
L-shaped dormer conversion
£75,000
£105,000
Common on Victorian terraces. Adds bedroom + ensuite.
Hip-to-gable + rear dormer
£75,000
£110,000
Standard on inter-war semis. Roof is restructured.
Mansard loft conversion
£90,000
£140,000
Full new roof structure. Best for terraces in conservation areas.
Double mansard (two storeys)
£140,000
£210,000
Rare. Requires Article 4 review and structural rebuild.
Regional cost variation
How regional cost varies (m² rates for the typical build).
Region
From
To
Notes
Prime central London (W1, SW1, SW3, NW1)
£132,000
£159,500
Premium finishes and listed-building consent standard.
Inner London (SE/SW/E/NW/N inside South Circular)
£110,000
£137,500
Strong Article 4 / conservation-area constraints.
Outer London (Bromley, Bexley, Croydon, Sutton, Havering)
£55,000
£110,000
Mid-band labour rates, PD typically intact.
Kent / Surrey / Essex commuter belt
£52,250
£121,000
Affluent areas (Sevenoaks, Tunbridge Wells, Weybridge) sit near top.
Outer Kent / Essex / Hertfordshire
£46,750
£104,500
Below-London labour costs; longer logistics.
What drives the cost
Roof shape and head height
A simple gable roof with ≥2.4 m centre headroom is cheapest. Hipped roofs add £8k–£15k for the hip-to-gable rebuild. Low-pitch roofs may need a full mansard, doubling the budget.
Staircase position
A new staircase has to start above a load-bearing wall. Where the only feasible position eats into a bedroom, expect an extra £4k–£8k of structural and joinery work.
Number of new rooms
One bedroom + ensuite is the typical scope. Adding a second bedroom (L-shape) adds £15k–£25k. A walk-in wardrobe or study nook costs little if it shares the same M&E run.
Insulation strategy
Part L 2022 sets stringent U-values. Warm-roof construction (rigid insulation above the rafters) costs £4k–£8k more than cold-roof, but eliminates condensation risk in modern airtight builds.
Glazing
Two standard Velux windows add £1,500. A full Juliette balcony with structural opening: £6k–£10k. Bespoke roof lantern: £8k–£15k.
Planning route
Permitted Development applies to most rear dormers. Conservation areas, listed buildings and Article 4 Directions force full planning, adding 8–12 weeks and £2k–£4k in fees.
Party walls
Almost every London loft conversion needs Party Wall Awards on both flanks. Surveyor cost is typically £1,200–£2,500 per neighbour.
What’s included
· Structural steel beams and engineer calcs
· Roof structure (dormer, hip-to-gable or mansard carpentry)
· Insulation to Building Regs Part L
· Fire-rated staircase, doors and detection
· Velux or dormer glazing
· First-fix electrics, plumbing and central heating extension
Loft conversions are typically priced per square metre of new internal floor — £2,800–£4,200/m² in London for 2026 — plus structural and stair allowances. A typical 30 m² L-shaped dormer prices out at £84,000–£126,000 on this basis, before second-fix bathroom and flooring. The cheapest way to control budget is to keep the new staircase landing simple and avoid moving the existing bathroom soil stack. The most common cost overrun is discovering that the existing ceiling joists are undersized — a 1980s extension might have 100×50 joists that need full sistering with new 220×47 timbers, adding £2k–£4k.
Loft conversion ROI and resale value
Estate agent data from across Greater London puts the average resale uplift of a well-executed loft conversion at 15–20% of the existing property value. On a £750,000 three-bed terrace, that's £112k–£150k of added value against a £75k–£105k build cost. The uplift is highest where the loft adds a fourth bedroom (i.e., turns a 3-bed into a 4-bed property), and lowest where it adds a study or playroom. Always confirm the local council tax band implications — loft conversions can push a property into the next band.
Choosing the right loft conversion type
For a typical Victorian terrace with a hip-and-gable rear, a rear dormer or L-shaped dormer is the right call — easy planning, good headroom, low risk. For inter-war semi-detached homes with hipped roofs, hip-to-gable is almost always the answer. For Georgian terraces, mansards are usually required by conservation officers because they preserve the original roof line from the front. Velux-only conversions are fine for studies and storage but rarely deliver the headroom for a full ensuite bedroom.
Where homeowners overspend
The two biggest overspends we see are bespoke glazing (lanterns, bifolds onto Juliette balconies) and structural changes to the floor below. If the loft conversion forces you to remove a load-bearing wall on the floor below to fit the new stair, that's £8k–£15k of additional structural work. Plan the stair early and avoid this. The third overspend is bathroom finishes — homeowners often spec a £15k bathroom in a £75k loft conversion. A £6k–£8k bathroom looks identical to a £15k one in photos.
Common loft conversion mistakes
Top mistakes from 18 months of RCB delivery: under-spec'd insulation that fails Part L at sign-off; ignoring the rest of the central heating system (which will struggle with the new radiators); cheap roof tiles that don't match the existing — buy reclaimed where possible; and skipping the Party Wall surveyor to "save money", which always backfires when the neighbour goes legal.
Frequently asked questions
How much does a loft conversion cost in London in 2026?
A typical rear-dormer loft conversion in London costs £55,000–£75,000 in 2026, before second-fix bathroom and flooring. L-shaped and hip-to-gable conversions sit at £75,000–£110,000. Mansards run £90,000–£140,000.
Is a loft conversion worth it financially?
Yes — typical resale uplift is 15–20% of the property value, well above the £55k–£110k build cost. The ROI is highest when the loft conversion adds a bedroom (i.e., turns a 3-bed into a 4-bed property).
Do I need planning permission for a loft conversion?
Most rear-dormer and Velux loft conversions sit inside Permitted Development. Mansards, side dormers in conservation areas, and listed buildings need full planning consent. Always get a Lawful Development Certificate to lock in PD status.
How long does a loft conversion take?
Typical timeline is 6–8 weeks of design and planning, then 8–12 weeks on site, then 1 week of snagging — so 16–20 weeks from sign-off to handover.
Can I stay in the house during the loft conversion?
Yes — most loft conversions are done from outside scaffold-up, keeping the existing roof in place until the new structure is weathertight. There will be 3–4 days of noisy structural work and 1 day of stair install where you may want to be out.
What's the cheapest loft conversion?
A Velux-only conversion on a roof with adequate head height costs £35k–£55k. It's suitable for a study, hobby room or fourth bedroom but rarely delivers full headroom across the whole floor.
Do I need a Party Wall Award for a loft conversion?
Almost always — installing steels into shared walls or excavating within 3 metres of a neighbour's foundations triggers the Act. Allow 4–8 weeks and £1,200–£2,500 per neighbour for the surveyor.
The numbers above are real averages — but every project is different. Use our 60-second instant quote, configure your project in detail, or book a free site visit for a fixed written quotation in 5 working days.