Cubic Metres, Not Square Metres: The 40 and 50 m³ Loft Conversion Allowance Explained, With the Numbers for Every Loft Type
By Anderson & the LCCL Construction Team | Finchley N3
Every guide to loft conversions, ours included, quotes two numbers in the same breath: the permitted development allowance of 40 cubic metres on a terraced house and 50 on a semi-detached or detached house, and the floor area a finished loft gives you — 20 to 30 square metres for a rear dormer, 30 to 40 for an L-shaped one. They are not the same measurement, they are not even the same kind of measurement, and on our consultation calls they are the thing people most often have the wrong way round. This is the explainer we wish existed when we started.
As LABC Building Excellence Award Winners 2024 for Best Residential Extension in London — and London Regional Winner 2026 for Best Conversion or Alteration to an Existing Home — we run the permitted development arithmetic on every loft we price, and our Planning check does it automatically for your postcode and house type. What follows is the rule, how it is counted, and the realistic figures for each loft, so you can see where your own house sits before anyone draws anything.
Not sure whether your project needs planning permission? Enter your postcode and tick what you want to build — about two minutes, free, and the from-price for each part that qualifies appears on screen and in your inbox.
The rule: a volume, measured on the outside
The permitted development right for roof extensions — Class B of Part 1 of the General Permitted Development Order — allows a house (not a flat) to enlarge its roof without a planning application provided the additional roof space does not exceed 40 cubic metres on a terraced house or 50 cubic metres on a semi-detached or detached house, and the other conditions are met: nothing higher than the existing ridge, no extension on the roof slope that faces a road, materials similar to the house, side windows obscure-glazed, and the dormer set back at least 200 mm from the eaves. The allowance is counted on the external dimensions of the new roof — the outside face of the dormer cheeks, the tiles and the flat roof, not the plasterboard inside — and it is cumulative: any roof extension that has been added to the house since it was built, by you or by a previous owner, counts against the figure. A house that already has a small rear dormer may have used half its allowance before you start.
Two things are not in the count. Rooflights that sit flush with the roof slope add no volume, which is why a rooflight-only loft is almost always permitted development even in a conservation area. And the existing loft space under the original roof is not counted either: the allowance is only the volume you add. That is the first reason the floor-area number and the volume number diverge — a loft that was always there as a void becomes a bedroom without using any of the allowance at all.
Floor area: what you live in and what you pay for
The square-metre figure is the usable floor, counted where there is standing headroom, and it is what the price is built on: the staircase, the floor structure, the insulation, the electrics and the finishes all scale with it. Our estimator sizes a loft by floor area for that reason. But floor area is a poor guide to the planning route, because most of the floor already exists under the old roof. A rear dormer on a terrace adds perhaps 30 to 40 cubic metres of roof volume and produces a 20 to 30 square metre room — most of that floor was there already; the dormer mainly adds headroom over it. A large L-shaped dormer on the same terrace can produce 35 to 40 square metres of floor, which sounds modest, while adding 45 cubic metres or more of roof, which exceeds the allowance.
The numbers for each loft type
These are typical figures for London terraces and semis; a surveyor's measurement of the actual roof settles the question for your house, and our Planning check does the arithmetic from your house type.
Velux / rooflight loft — adds 0 m³ (rooflights flush with the slope). Floor: 15–25 m² under the existing roof. Permitted development on almost every house, conservation areas included.
Rear dormer (main roof only) — adds about 30–40 m³. Floor: 20–30 m². Inside the 40 m³ terrace allowance on most houses, comfortably inside 50 on a semi.
Hip-to-gable with a rear dormer — adds about 45–50 m³ (the gable fill-in is most of it). Floor: 25–35 m². Fits the 50 m³ semi or detached allowance on most 1930s houses; on an end-of-terrace, which counts as a terrace at 40 m³, it usually does not.
L-shaped dormer (main roof + outrigger) — adds about 40–55 m³ depending on the outrigger length. Floor: 30–40 m². The one that most often tips over: a compact L-shape on a terrace fits inside 40 m³, a full-length one does not and needs a householder application.
Mansard — adds 60 m³ or more and changes the roof shape and eaves height. Floor: 30–45 m². Never permitted development; always a planning application.
Read across the list and the pattern is the one that confuses people: the floor area climbs gently from 20 to 45 square metres while the volume climbs steeply from zero to sixty cubic metres. The council is looking at the right-hand column; you are living in the left-hand one.
Why the two allowances are 40 and 50
A terraced house is counted as any house with another house attached on both sides, and an end-of-terrace counts as terraced, not semi-detached — a point that catches people out, because an end-of-terrace looks and often behaves like a semi but gets the smaller 40 cubic metre allowance. A semi-detached house is attached on one side only; a detached house on none. The larger allowance for semis and detached houses reflects their hipped roofs, which need more added volume to make a usable floor at all: a hip-to-gable spends 20 cubic metres or so simply turning the hipped end into a vertical gable before the dormer adds a single square metre of standing room.
When the arithmetic tips a loft into planning
Three situations account for nearly every loft we price that needs an application rather than a certificate. A full-length L-shaped dormer on a terrace, where the outrigger dormer adds ten to fifteen cubic metres on top of a main-roof dormer that already uses thirty. A hip-to-gable on an end-of-terrace, where the 40 cubic metre allowance applies instead of 50 and the gable fill-in alone is most of it. And a house with an earlier roof extension — a small dormer, a raised roof over the outrigger — that nobody mentioned, where the cumulative total is reached before the new work starts. In each case the answer is not to shrink the loft until it fits; it is to decide which is worth more to you, the extra room or the certificate. A householder application (£548, eight weeks) for a loft that is slightly over the allowance is approved far more often than refused outside conservation areas, because the council is judging appearance, not volume, once the application is in.
The reverse mistake is as common. Owners who have read that a loft "must be under 40 square metres" assume their 32 m² rear dormer needs planning and pay for an application they do not need. A Lawful Development Certificate (£274) is the right route for a loft inside the allowance, and it is the certificate a buyer's solicitor will ask for later.
Conservation areas change the rule, not the arithmetic
Inside a conservation area — and in Article 4 streets — the roof-extension right under Class B is withdrawn altogether, so the 40 and 50 cubic metre figures stop mattering: any dormer or hip-to-gable there is a householder planning application whatever its volume. Rooflights usually remain permitted development because they add none. Our Planning check reads the conservation-area and Article 4 data for your postcode first, then applies the volume rule only where it still applies; the council has the final say on any application.
Not sure whether your project needs planning permission? Enter your postcode and tick what you want to build — about two minutes, free, and the from-price for each part that qualifies appears on screen and in your inbox.
Anderson's advice
Measure the roof before you fall in love with a drawing. The floor plan an architect shows you is drawn from the inside; the council measures from the outside, and the difference between a 38 cubic metre dormer and a 44 cubic metre one is a few hundred millimetres of cheek height that no one notices on a plan. Our team measures the existing roof at the survey, works out the volume of the proposed dormer on the outside face, and tells you the number before the drawings are finished — if it is close to the line, we draw it 2 cubic metres under, not 2 over, because a Lawful Development Certificate is worth more at resale than the extra shelf of headroom. And on a semi, a hip-to-gable with a rear dormer is almost always inside 50; talk to the neighbour early, because the party wall is what moves the date, not the planning.
What our clients say
“used LCCL construction for my loft conversion. Very happy with the works, arranged everything from planning to finish. completed on time with no hassle.”
— Brian Edwards, Google Reviews
Frequently asked questions
Is the 40 cubic metre loft allowance a floor area?
No. It is the volume of roof space you add, measured on the outside of the new roof, and it does not include the loft void that was already there under the original roof. A 40 cubic metre dormer typically gives 20 to 30 square metres of floor, because most of the floor existed before the dormer added headroom over it.
Does an end-of-terrace house get 40 or 50 cubic metres?
40. An end-of-terrace counts as a terraced house under the permitted development rules, not a semi-detached one, so the smaller allowance applies. A hip-to-gable with a rear dormer on an end-of-terrace usually needs a householder planning application for that reason.
Do previous roof extensions count against the allowance?
Yes. The allowance is cumulative for the house since it was built, so a dormer or raised roof added by a previous owner reduces what is left. Rooflights flush with the slope do not count. Our Planning check asks whether the house has been extended before for exactly this reason.
Check your own house
Not sure whether your project needs planning permission? Enter your postcode and tick what you want to build — about two minutes, free, and the from-price for each part that qualifies appears on screen and in your inbox.
To talk through the volume arithmetic for your loft, book a free 30-minute consultation call with LCCL Construction on 020 8037 7332, or get an instant estimate by floor area in about two minutes.
One team, one contract, one point of contact — from first sketch to handover.
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