Is an end-of-terrace house 40m³ or 50m³?

40m³. Class B.1(d) sets the lower limit for a terrace house, and an end-of-terrace is a terrace house — it shares at least one party wall and forms part of the terrace. This is one of the most common and most expensive misreadings of the allowance, because a scheme designed to 50m³ on an end-of-terrace is 10m³ over and has no planning permission at all.

Does a rooflight conversion use up any of the allowance?

No. Rooflights are permitted under Class C, which adds no volume to the roof space at all. The 40m³/50m³ figure is a Class B limit and a rooflight conversion never engages it.

Does a previous owner's dormer count against my allowance?

Yes. The allowance is measured against the roof of the original dwellinghouse — as first built, or as it stood on 1 July 1948 if older. Every enlargement since then counts, regardless of who carried it out or when. If a previous owner added a 20m³ dormer to a semi-detached house, you have 30m³ left, not 50m³.

Do I measure the internal room or the external volume?

External. The calculation is the cubic content added above the plane of the original roof, measured externally — so the dormer's cheeks, roof build-up and structure are all inside the figure. The finished room you end up with is always smaller than the volume you have spent.

Planning & Permitted Development

The 40m³ / 50m³ volume allowance, explained

Permitted Development allows 40m³ of additional roof volume on a terraced house (including end-of-terrace) and 50m³ on a semi-detached or detached house. It is measured against the original roof, it is cumulative across every previous enlargement, and it is the volume added above the old roof plane — not the size of the finished room.

Last reviewed 1 September 2026

The two numbers

GPDO 2015, Schedule 2, Part 1, Class B.1(d).
House type	Volume allowance
Terraced house	40m³
End-of-terrace house	40m³
Semi-detached house	50m³
Detached house	50m³
Flat or maisonette	No Permitted Development rights at all

What “original” means

The allowance is measured against the original dwellinghouse: the house as it was first built, or as it stood on 1 July 1948 if it was built before that date. It is emphatically not the house as you bought it.

This has a consequence people rarely discover until they are already designing: if any previous owner enlarged the roof, that volume has already been spent, and it is spent from your allowance. A 1990s rear dormer on a semi-detached house that consumed 22m³ leaves the current owner 28m³, and no amount of it having happened before they moved in changes that.

How the volume is actually calculated

The figure is the cubic content added above the plane of the original roof, measured externally. For a standard rear dormer with a flat roof, the shape you are measuring is a wedge: bounded below by the original roof slope, above by the dormer’s flat roof, and on three sides by its cheeks and front face.

The three dimensions that drive it

Width — how far the dormer runs along the roof. The biggest single lever, and the one most constrained by the eaves setback and, on a hipped roof, by where the hip plane starts.
Depth — how far it projects back toward the ridge. Limited by the 200mm eaves setback at one end and by the “nothing above the ridge” rule at the other.
Height — the vertical distance between the original roof plane and the dormer’s flat roof, which grows with depth on a pitched roof. This is why volume rises faster than floor area: a dormer twice as deep is considerably more than twice the volume.

Spending the allowance well

On most roofs the allowance is not the binding constraint — the available head height and the stair are. But on wide semi-detached roofs, and on any house that has been extended before, it becomes the thing that decides the scheme. Two rules of thumb:

A hip-to-gable is expensive in volume terms — typically 15–25m³ — but it buys usable floor area at full height rather than dead space. If you are doing both a hip-to-gable and a rear dormer on a 50m³ allowance, expect to be close to the cap and to need the numbers checked rather than estimated.
If you are within about 5m³ of the limit, estimated dimensions are no longer good enough. At that margin the difference between a tape measure and a measured survey is the difference between a lawful conversion and an enforcement problem.

Checking your own figure

LoftNow’s free assessment computes the added volume from your roof dimensions and dormer configuration, subtracts any prior roof alterations you tell us about, and checks the result against the correct cap for your house type. The paid report shows the full working — every dimension and formula, lettered onto a 3D wireframe of your roof — which is the form a council wants to see it in if you go on to apply for a Lawful Development Certificate.

Common questions

Is an end-of-terrace house 40m³ or 50m³?: 40m³. Class B.1(d) sets the lower limit for a terrace house, and an end-of-terrace is a terrace house — it shares at least one party wall and forms part of the terrace. This is one of the most common and most expensive misreadings of the allowance, because a scheme designed to 50m³ on an end-of-terrace is 10m³ over and has no planning permission at all.
Does a rooflight conversion use up any of the allowance?: No. Rooflights are permitted under Class C, which adds no volume to the roof space at all. The 40m³/50m³ figure is a Class B limit and a rooflight conversion never engages it.
Does a previous owner's dormer count against my allowance?: Yes. The allowance is measured against the roof of the original dwellinghouse — as first built, or as it stood on 1 July 1948 if older. Every enlargement since then counts, regardless of who carried it out or when. If a previous owner added a 20m³ dormer to a semi-detached house, you have 30m³ left, not 50m³.
Do I measure the internal room or the external volume?: External. The calculation is the cubic content added above the plane of the original roof, measured externally — so the dormer's cheeks, roof build-up and structure are all inside the figure. The finished room you end up with is always smaller than the volume you have spent.

Sources

GPDO 2015 — Schedule 2, Part 1, Class B — the roof-enlargement rules, including the 40m³/50m³ volume allowance
Permitted development rights for householders: technical guidance — the government's plain-English reading of Class B, with diagrams

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Related guides

Permitted Development rules for loft conversions

Class B and Class C of the GPDO 2015, explained clause by clause: volume limits, ridge height, eaves setback, highway frontage, materials and side windows.

Rear dormer loft conversion rules

The Permitted Development limits that shape a rear dormer: volume, ridge height, the 200mm eaves setback, highway frontage, matching materials and side windows.

Hip-to-gable loft conversions

Squaring off a hipped roof end to a vertical gable. What it costs in Permitted Development volume, which houses can have one, and when to pair it with a rear dormer.

Do I need planning permission for a loft conversion?

Most loft conversions in England need no planning permission — they are Permitted Development. Here are the six conditions that decide it, and the cases that fail.

Written by the LoftNow team from the legislation and Approved Documents cited above, and reviewed against the rules our assessment engine implements. It is general guidance for properties in England, not advice on your particular property — Permitted Development is self-assessed, and the definitive answer for a specific house comes from your local planning authority, normally via a Lawful Development Certificate.