Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
Loft conversions, house extensions, renovations, basements and new builds across London and Hertfordshire.
Services
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Loft conversion types · London & Hertfordshire
Loft conversion types: which one suits your roof, and what each costs in 2026
By Anderson & the LCCL Construction Team
Finchley N3 · Design & build, LABC award winners 2024 and 2026
There are five loft conversions and your roof usually chooses for you: a gable roof on a terrace wants a dormer or an L-shaped dormer, a hipped roof on a semi wants a hip-to-gable, a period terrace in Central London wants a mansard, and a tall roof with a modest budget can take Velux windows alone. This page compares all five — cost, space, planning and build time — then sends you to the detailed page for the one that fits.
Quick answer
£32,400 – £148,800 inc. VAT (£27,000 – £124,000 + VAT) design and build inclusive, depending on the type: Velux from £32,400 inc. VAT (£27,000 + VAT), dormer from £56,400 inc. VAT (£47,000 + VAT), hip-to-gable from £60,000 inc. VAT (£50,000 + VAT), mansard from £76,800 inc. VAT (£64,000 + VAT).
Velux, dormer, L-shaped and hip-to-gable conversions are usually permitted development. A mansard almost always needs a planning application.
4–16 weeks on site depending on the type; 8–16 weeks of design and approvals before that.
A free 30-minute phone call with our team, no obligation.
Guide price
£32,400 – £148,800 inc. VAT (£27,000 – £124,000 + VAT)
Typical build
4–16 weeks on site
Guarantee
12-month workmanship guarantee; 10-year LABC structural warranty
Awards
LABC 2024 & 2026 winners
LABC Building Excellence Award 2024
Best Residential Extension in London — Bohun Grove, East Barnet EN4: rear extension, hip-to-gable loft conversion and whole-house renovation
LABC Building Excellence Awards 2026
London Regional Winner, Best Conversion or Alteration — Weech Road, Hampstead NW6: penthouse loft conversion and full renovation
One contract
Architecture, structural engineering, planning, construction and project management — all under one contract
Guaranteed & insured
12-month workmanship guarantee; 10-year LABC structural warranty
What every LCCL loft conversion includes
The same design and build package whichever type you choose.
Measured survey of the roof space, stairs and existing structure
Architectural design and drawings, with revisions until you're happy
Lawful Development Certificate or planning application, prepared and submitted for you
Building regulations drawings and structural calculations by our chartered engineer
Party wall notices served on your behalf (£300 each, at cost)
New floor structure, steels, dormer or gable structure, roof and windows
Staircase, insulation, fire doors and escape windows to building regulations
Plaster finish, first-fix electrics and plumbing; bathroom or en-suite where specified
Second-fix carpentry, decoration and the items you supply fitted
LABC inspections and completion certificate
A dedicated project manager and a weekly progress update
12-month workmanship guarantee; 10-year LABC structural warranty
How much does each loft conversion type cost in London in 2026?
All five types on one table, design and build inclusive. Every project is priced from your postcode and roof.
Loft conversion type
What it gives you
Guide range
Velux / rooflight
Roof kept, windows in the slope — cheapest, needs a tall roof
£32,400 – £50,400 inc. VAT (£27,000 – £42,000 + VAT)
Box on the rear slope, full-height bedroom and en-suite
£56,400 – £80,400 inc. VAT (£47,000 – £67,000 + VAT)
Rear dormer plus outrigger dormer — two rooms on a terrace
£58,800 – £98,400 inc. VAT (£49,000 – £82,000 + VAT)
Hipped side rebuilt as a gable, usually with a rear dormer — the semi answer
£60,000 – £114,000 inc. VAT (£50,000 – £95,000 + VAT)
New roof across the footprint — most space, usually planning
£76,800 – £148,800 inc. VAT (£64,000 – £124,000 + VAT)
Build time
On site
Planning
Velux / rooflight
4–6 wks
Permitted development
Rear dormer or L-shaped dormer
8–12 wks
Permitted development within 40/50 m³
Hip-to-gable with dormer
10–12 wks
Permitted development within 50 m³
Mansard
12–16 wks
Planning application
What the price includes: survey, drawings, lawful development certificate or planning, building-regulations drawings, structural calculations, new floor structure, dormer or gable structure, roof and windows, stairs, insulation, fire doors and escape windows, first and second fix, plastering, decoration, a bathroom where specified, LABC certificate and guarantees. Not included unless specified: sanitaryware above the allowance, bespoke joinery, party wall surveyor fees if a neighbour dissents (from £750). The figure on your itemised estimate is the figure you pay — nothing is added afterwards. Postcode dependent — the instant estimator adjusts the range for your area in thirty seconds, and a detailed itemised estimate follows one survey.
Why the types cost what they do: Velux is cheapest because the roof structure stays — you pay for floor, stairs, insulation and windows. A dormer adds a box structure and a flat roof. Hip-to-gable rebuilds a whole roof slope, so it costs more but gives the biggest room on a semi. A mansard is a new roof on almost the whole footprint — the most space and the most money, and usually a planning application. Inner and Central London (Camden, Islington, Hackney, Wandsworth, Hammersmith, Westminster, Kensington & Chelsea, the City) cost more than outer London and Hertfordshire a little less — the instant estimate applies your postcode. These are estimated ranges — every home is surveyed and costed individually.
Which type suits which house?
Planning
Which loft conversions need planning permission?
Four of the five are usually permitted development. The mansard is the exception.
Route
Conditions
Time
What we do
1. Permitted development
Rear dormer or hip-to-gable within 40 m³ (terrace) / 50 m³ (semi or detached), no higher than the ridge, set back 20 cm from the eaves, no front dormers on the principal elevation
No application
Lawful Development Certificate, ~8 weeks, included
2. Full planning
Mansards, flats, conservation and Article 4 areas, front dormers, roof raised above the ridge, volume over the limit
8 weeks
Drawings, design statement and application by our team
3. Listed building consent
Listed houses — every external and most internal changes
8 weeks
Heritage statement and application alongside planning
Building regulations & party walls
The other two approvals
Every loft conversion needs building-regulations approval whether or not it needs planning: the new floor, the steels, the stairs, insulation, fire doors and escape windows. We do this through LABC with full-plans drawings and our own structural calculations, so the inspector is checking a design he has already approved.
On a semi or terrace the Party Wall Act applies because the new steels bear on the shared wall. Notices go out at least two months before we start; most neighbours consent, and where a surveyor is appointed we manage it.
The volume allowance — 40 m³ on a terrace, 50 m³ on a semi or detached house — is measured on the survey and counts every roof enlargement since the house was built. We obtain a Lawful Development Certificate on every permitted development loft; it is the document that proves the conversion was lawful when you sell.
Dormer vs hip-to-gable vs mansard · Loft conversion planning permission
Recent loft conversion projects
Bohun Grove, East Barnet EN4 — rear extension, hip-to-gable loft and whole-house renovation
LABC 2024 Best Residential Extension in London
Semi-detached family home One programme loft + extension + renovation
A total renovation of a family semi: a rear extension opening into a full-width kitchen and dining space with floor-to-ceiling glazing and skylights, a hip-to-gable loft conversion with a rear dormer, two Lusso Stone bathrooms, enlarged bedrooms, new windows throughout, a regulated outbuilding and landscaping.
Because the loft and extension ran under one contract, the steelwork for the open-plan ground floor and the loft floor was designed together and lifted in on the same day — one crane hire, one scaffold, one building-control file.
See the Bohun Grove project · All projects
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ANDERSON · FOUNDER, LCCL CONSTRUCTION
Anderson's advice: talk to next door before the scaffold goes up
Semi-detached hip-to-gable and rear dormer combinations almost always fall under permitted development, but neighbours often object informally — to the council, to the building inspector, to you. Talk to them before the scaffold goes up. We will show them the drawings with you.
It costs nothing, and it is the difference between a quiet twelve weeks and a formal complaint that stalls the job.
How a loft conversion runs with LCCL
From the first call to the completion certificate. Design and approvals typically take 8–12 weeks before work starts; the scaffold goes up on day one and the roof is made weather-tight in the first fortnight.
WEEK 0
Free consultation call
A 30-minute phone call with our team about the house, what you want from the space and a realistic budget. Your instant estimate is the starting point.
WEEKS 1–4
Survey & design
Measured survey, architectural drawings and your detailed itemised estimate — design, planning, structure, construction and warranty priced line by line.
WEEKS 4–12
Approvals
Lawful Development Certificate or planning application, building-regulations drawings, structural calculations, party wall notices. Nothing starts on site until it is signed off.
BUILD
On site
4–16 weeks on site for a loft conversion, depending on the type. Weekly updates from your project manager, one point of contact throughout.
HANDOVER
Sign-off
Snagging, LABC final inspection and completion certificate, guarantees in writing. 10-year LABC structural warranty.
“Extension built by LCCL Construction. We have just recently completed our ground floor house extension on our semi-detached home in Finchley. The services we used were for the design and build of the extension which is three meters and the full breadth of…”
EVA HALL · Trustpilot
“We are very happy with the services from Lccl, Alex has done a great job on design and construction. We have used Lccl in the past for our rear house extension and have come back to instruct Lccl for our loft conversion too to our house in North London. As…”
Mia M · Google Reviews
“Everyone at LCCL Construction was skilled, trustworthy and worked to the highest standard. I am delighted with our loft conversion, it is exactly what we wanted and I highly recommend the company - they have transformed the quality of my home. Overall it…”
KashImaging · Google Reviews
Loft conversion type questions we get asked most
A Velux or rooflight conversion, from £32,400 inc. VAT (£27,000 + VAT) in 2026, because the roof structure stays and you pay for the floor, stairs, insulation and windows. It needs a tall roof — about 2.4 m to the ridge — to give a usable room without a dormer.
A mansard, because it replaces the roof with near-vertical walls and a flat top across almost the whole footprint. It is also the most expensive (£76,800 – £148,800 inc. VAT (£64,000 – £124,000 + VAT)) and almost always needs planning. On a semi, the hip-to-gable with rear dormer is the practical maximum under permitted development.
Look at the roof from the street. A gable end (vertical wall to the ridge) means a dormer or L-shaped dormer; a hipped end (sloping on all sides) means a hip-to-gable; a period terrace with mansards next door means a mansard. The survey confirms the head height and the volume allowance.
The usual test is 2.2 m from the top of the ceiling joists to the underside of the ridge. Most Victorian, Edwardian and 1930s houses pass it; some 1960s and later houses with shallow trussed roofs do not, and need the roof raised — which is a planning application and priced separately.
Usually not. Velux, dormer, L-shaped and hip-to-gable conversions within 40 m³ (terrace) or 50 m³ (semi/detached) are permitted development and we obtain a Lawful Development Certificate. Mansards, flats, conservation and Article 4 areas and front dormers need a planning application.
Four to six weeks on site for a Velux loft, eight to twelve for a dormer or L-shaped dormer, ten to twelve for a hip-to-gable, twelve to sixteen for a mansard, plus 8–16 weeks of design and approvals before that.
Yes — and it is usually cheaper than doing them a year apart because the scaffold, structural design and party wall process are shared. Bohun Grove EN4, our LABC 2024 award winner, was a hip-to-gable loft and a rear extension in one programme. See our loft + extension page.
A bedroom and bathroom in the roof is one of the improvements London buyers pay most for. Our loft-conversion value guide sets out what the data shows by area. We are not valuers, so treat any figure as indicative and talk to a local agent before you commit.
Where we build loft conversions
From Finchley N3 we cover North London, North-West London, Central London, South-West and West London, and Hertfordshire — and we also work across the rest of London, Buckinghamshire, Surrey, Kent and Essex. Recent lofts: Hampstead NW6, East Barnet EN4, Putney SW15, Wood Green N22, Borehamwood WD6, Hendon NW4 and Marylebone W1.