Permitted Development Loft Conversion in London: The Rear Dormer You Can Build Without Planning Permission
By Anderson & the LCCL Construction Team | Finchley N3
A permitted development loft conversion is one that fits inside the rules Parliament has already approved for every house in England, so there is no planning application, no committee and no neighbour consultation to wait for. For most terraced and semi-detached houses in London that means a rear dormer: up to 40 cubic metres of new roof space on a terrace, 50 on a semi or detached house, built to a set of design limits and confirmed with a Lawful Development Certificate from the council.
With interest rates where they are, it is the loft most of our clients are asking for in 2026: a double bedroom and shower room in 6–8 weeks on site, at a fixed price, with the planning risk removed before a deposit is paid. This guide covers what permitted development allows, what it does not, how to tell whether your house qualifies, and what we charge at economy specification.
What permitted development allows in the roof
The rules are Class B of the General Permitted Development Order. In plain English, a loft conversion is permitted development when all of the following are true:
Volume: the new roof space is no more than 40 m³ on a terraced house or 50 m³ on a semi-detached or detached house. Any previous extension to the roof — a dormer added by an earlier owner — counts against the allowance.
Height: nothing goes above the highest part of the existing roof (the ridge).
Front: no dormer on the roof slope facing the road. Rooflights that sit flush are fine.
Set-back: the dormer sits at least 20 cm back from the original eaves, measured along the roof slope.
Materials: similar in appearance to the existing house — slate or tile-hung cheeks on a slate roof, for example.
Side windows: obscure-glazed, and non-opening unless the opening part is more than 1.7 m above the floor.
No balcony, veranda or raised platform — a roof terrace takes the whole scheme out of permitted development.
If the house is a flat or maisonette, in a conservation area, or on a street with an Article 4 direction removing permitted development rights, Class B does not apply and the loft needs a full application. Our guide to whether a loft conversion needs planning permission in London goes through those exceptions borough by borough.
What 40 or 50 cubic metres actually gives you
A typical London rear dormer on a two-storey terrace uses 30–38 m³ and produces a loft of 20–30 m² of usable floor — a double bedroom with a shower room, or a large bedroom with eaves storage. On a semi the extra 10 m³ is what makes a hip-to-gable with rear dormer possible, and on a terrace with a back addition an L-shaped dormer over the main roof and the outrigger uses the allowance to the full. If you only need light and a small room, a Velux loft uses almost none of the allowance at all.
The Lawful Development Certificate
Permitted development does not need anyone's permission, but it does need proof — when you sell, the buyer's solicitor will ask for it. A Lawful Development Certificate is the council's written confirmation that the loft, as drawn, is lawful without planning permission. We apply for one on every permitted development loft we build: the drawings go in with the application, the council has eight weeks to decide, and the certificate is yours to keep. It costs a fraction of a planning application and it is included in our price.
What a permitted development rear dormer costs in 2026
Our figures are inc. VAT with the ex-VAT amount in brackets, at economy specification (shell and plaster, with the bathroom fittings and a fully finished level priced as separate lines) and postcode dependent:
Rear dormer loft conversion: from £56,400 inc. VAT (£47,000 + VAT), typically £56,400 – £80,400 (£47,000 – £67,000 + VAT) for 20–30 m².
Included: structural design and calculations, building regulations drawings and full-plans submission, the Lawful Development Certificate application, party wall notices, the new floor, dormer, stair, insulation and fire protection, first and second fix to the finish level chosen.
Priced separately: bathroom fittings (an allowance at economy specification), a fully finished level, and a party wall surveyor's fees if a neighbour dissents.
The figure is fixed for the scope on the drawings. The dormer loft conversion guide breaks the build down stage by stage, and the instant estimate gives you a range for your postcode in two minutes.
Anderson's advice
“Semi-detached hip to gable + rear dormer combos almost always fall under permitted development, but neighbours often object informally.” The same is true of a plain rear dormer on a terrace. Permitted development means the neighbour cannot stop it, but they will still see the scaffold and hear the work, and the party wall notice still has to be served and agreed. We knock on the door before the notice goes through it. A five-minute conversation about dates and dust has saved more of our start dates than any document.
Why clients choose us for permitted development lofts
As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winners again in 2026 for Best Conversion or Alteration to an Existing Home — our 2026 award was for a loft — the top floor at Weech Road NW6 turned into a penthouse — and it was nominated by the building control surveyor who inspected it. One contract covers the design, the certificate, the structure and the build, with a 12-month workmanship guarantee and a 10-year LABC structural warranty. Finchley N3 based, covering London and Hertfordshire. See the Weech Road project.
What our clients say
“I highly recommend LCCL Construction. They completed our loft conversion to a very high standard, and the quality of workmanship throughout was excellent. The team was professional, reliable, and completed everything within a very reasonable timeframe. They were also fairly priced, which made the whole experience even better. We’re really pleased with the finished result and would definitely use them again.”
— Alina A, Google Reviews
Frequently asked questions
Do I need planning permission for a loft conversion in London?
Usually not. A rear dormer within 40 m³ on a terrace or 50 m³ on a semi or detached house, below the ridge, set back 20 cm from the eaves and in similar materials is permitted development. Flats, conservation areas, Article 4 streets, front dormers and roof terraces need a planning application.
What is a Lawful Development Certificate and do I need one?
It is the council's written confirmation that your loft is lawful without planning permission. It is not compulsory, but a buyer's solicitor will ask for it when you sell, so we apply for one on every permitted development loft and include it in the price.
How much does a permitted development loft conversion cost in London?
A rear dormer is from £56,400 inc. VAT (£47,000 + VAT) at economy specification, typically £56,400 – £80,400 inc. VAT for 20–30 m², with bathroom fittings and a fully finished level priced separately. Figures are postcode dependent.
Get a figure for your own project
To book a free 30-minute consultation call about your permitted development loft conversion, call LCCL Construction on 020 8037 7332 or get an instant estimate online — six questions, about two minutes, and you see a real range for your postcode on screen. Finchley N3 based, covering all of London and Hertfordshire.
One team, one contract, one point of contact — from first sketch to handover.
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