Hip to gable loft conversions and the 50 cubic metre limit
Class B allows 40 cubic metres on a terrace house and 50 otherwise, measured against the original roof. Filling one hip end takes a large share of it.
A hip to gable loft conversion is permitted development on many houses, but the volume allowance is tighter than most builders suggest. Class B of Schedule 2 Part 1 to the General Permitted Development Order 2015 allows the roof space to grow by up to 40 cubic metres on a terrace house and 50 cubic metres on any other house, measured against the roof as originally built. Filling in one hip end swallows a surprising share of that, and filling in two usually leaves too little for the rear dormer the conversion needs.
What Class B allows
Class B covers additions and alterations to the roof of a house. A hip to gable, a rear box dormer, a side dormer and a roof enlargement joining the house to an extension all sit inside it.
Five limits apply, and failing any one of them takes the whole roof enlargement outside Class B.
- No part of the enlargement may exceed the height of the highest part of the existing roof.
- No part may extend beyond the plane of any existing roof slope that forms the principal elevation and fronts a highway, which is why front dormers almost always need an application.
- The cubic content of the resulting roof space must not exceed that of the original roof space by more than 40 cubic metres for a terrace house or 50 cubic metres otherwise.
- No veranda, balcony or raised platform.
- The house must not be on designated land.
A terrace house for these purposes means a house in a row of three or more, sharing a party wall or an adjoining main wall on both sides, or on one side for an end of terrace. A semi-detached house sits in a row of two, so it takes the 50 cubic metre allowance rather than 40.
How the volume is measured
The measurement is of roof space volume, taken externally, so the thickness of the roof structure counts. Measure the original roof space, measure the proposed roof space, and the difference is what counts against the allowance.
"Original" means the house as built, or as it stood on 1 July 1948 if it is older. Every earlier roof enlargement therefore counts against your allowance, including a side dormer put in 30 years ago by a previous owner, whether or not it needed permission at the time. The allowance is not refreshed by a change of ownership and it is not refreshed by time.
A note on habitable volume. The allowance is about roof space, not about the floor area you gain. A conversion can use 30 cubic metres of allowance and deliver a room you cannot stand up in at the edges, which is why the headroom calculation and the volume calculation should be done together at sketch stage.
Why a hip to gable eats the allowance
The volume of the wedge you add when a hipped end becomes a vertical gable follows from the roof geometry, and it is worth computing before the design goes further.
The added volume is a pyramid: its base is the new triangular gable, and its apex is the point where the old hip ridge ended. For a roof of span S measured front to back, a ridge height H above eaves level and a hip that slopes back by a horizontal distance D, the added volume is approximately S multiplied by H multiplied by D, divided by six.
Take an illustrative example. A detached inter-war house with a span of 8 metres, a 40 degree pitch and so a ridge roughly 3.3 metres above the eaves, with the hip sloping back about 4 metres. That gives roughly 18 cubic metres for one hip end. Two hip ends on a detached house take about 36 cubic metres, leaving around 14 cubic metres of the 50 for everything else. These figures are illustrative only, derived from the geometry above, and your own roof needs measuring from the drawings.
That arithmetic explains the pattern in refused prior approvals and enforcement cases. A detached house with two hips, converted to gables, with a full-width rear dormer, is almost never within Class B. An end of terrace with one hip, converted to a gable, with a modest rear dormer, usually is.
| House type | Volume allowance | Typical outcome |
|---|---|---|
| Mid terrace (usually no hip) | 40 m³ | Rear dormer alone, usually within the allowance |
| End of terrace, one hip | 40 m³ | One gable plus a modest rear dormer, tight |
| Semi-detached, one hip | 50 m³ | One gable plus a rear dormer, usually workable |
| Detached, two hips | 50 m³ | Both gables plus a dormer, rarely workable |
Adding a rear dormer to a gable conversion
The rear dormer is where the remaining allowance goes, and its volume depends on three things: width, how far the front face projects from the original roof plane, and its height.
Reduce any one of them and the volume falls quickly. Pulling a full-width dormer in by half a metre at each side is often enough to bring a scheme back inside the allowance, and it usually improves the appearance from the garden as well.
Where the numbers will not work, the choice is between a smaller roof and a planning application. A householder application for a loft conversion is a short pack, decided in eight weeks for a minor development, and a council that would have accepted the scheme anyway is not made hostile by the application. Put the volume calculation in the Planning Statement so the officer can see you understand why permission is needed, and set out the design reasoning for the dormer proportions.
Materials, windows and the eaves set back
Three conditions attach to Class B rather than limits, and they are easy to breach by accident.
Materials must be of similar appearance to those used on the exterior of the existing house. Tile profile and colour matter more than anything else on a gable, because the new gable wall is a large, visible plane. Hanging tiles, render or boarding on a gable that was previously a tiled hip changes the appearance of the house, and whether that counts as similar in appearance is a question your council may take a different view on. The same reasoning governs rendering or cladding a house.
Side-facing windows in the enlargement must be obscure-glazed and non-opening, unless the opening parts are more than 1.7 metres above the floor of the room.
The 20 centimetre eaves set back is the condition people get wrong in the useful direction. The enlargement normally has to sit at least 20 centimetres back from the eaves, measured along the roof slope. That requirement expressly does not apply to a hip to gable enlargement, or to an enlargement joining the original roof to the roof of a rear or side extension. So the gable can run down to the eaves line, while a rear dormer on the same roof still has to be set back.
Designated land and flats
Class B does not apply at all on designated land: conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads. A hip to gable in a conservation area is a planning application from the start, and the submission will usually need a Heritage Statement assessing the effect on the character and appearance of the area. Roofscape is one of the things conservation area appraisals most often identify as a defining feature, so expect the gable itself to be the issue rather than the dormer.
Class B also does not apply to flats or maisonettes. Part 1 rights belong to dwellinghouses, and a top-floor flat has none of them, however the roof is owned.
An article 4 direction can remove Class B in a specific area even without a designation. Check the authority's mapping before relying on the allowance, because article 4 directions aimed at roof alterations are common in inter-war suburbs with consistent roofscapes. The same checking discipline applies to the principal elevation question that makes front extensions harder than rear ones.
Proving what was there before
If the allowance has been partly used, you need to know by how much, and the burden of proof is yours.
Start with the council's online planning history for the address. Then historic Ordnance Survey mapping and aerial imagery, which will show a side dormer or a rear box that was never applied for. Then the deeds plan and any original sales particulars. Where a previous enlargement was unauthorised and is now immune from enforcement, it still counts against the volume allowance, which is a point that surprises sellers and buyers alike.
A lawful development certificate under section 192 of the Town and Country Planning Act 1990 is the only formal way to confirm that the proposed works would be lawful without permission. It costs a fraction of a full application fee and it is the document a conveyancer will ask for in five years' time.
Enter your postcode on PlanScope to see which reports a loft conversion on your house is likely to need.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Is a hip to gable loft conversion permitted development?
Often, under Class B of Schedule 2 Part 1 to the General Permitted Development Order 2015, provided the enlargement stays within the volume allowance, does not exceed the existing ridge height, does not break the plane of the principal elevation roof slope, and the house is not on designated land or covered by an article 4 direction.
Does a semi-detached house get 40 or 50 cubic metres?
Fifty. A terrace house means a house in a row of three or more sharing party or adjoining main walls on both sides, or on one side for an end of terrace. A semi-detached house sits in a row of two, so it falls outside that definition and takes the 50 cubic metre allowance.
Do earlier dormers count against the allowance?
Yes. The comparison is against the roof space as originally built, or as it stood on 1 July 1948 for older houses. Any previous roof enlargement counts, including one built by a previous owner and one that was unauthorised but is now immune from enforcement. The allowance does not reset on sale.
Does the 20 centimetre eaves set back apply to a gable?
No. The condition requiring the enlargement to sit at least 20 centimetres back from the eaves, measured along the roof slope, expressly excludes a hip to gable enlargement and one joining the original roof to a rear or side extension. A rear dormer on the same roof still has to be set back.
Can I do a hip to gable in a conservation area?
Not under permitted development. Class B does not apply on designated land, which includes conservation areas, National Parks, National Landscapes, World Heritage Sites and the Broads. You will need a householder application, usually with a heritage statement, and the gable itself is likely to be the main issue rather than the dormer.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.