Hip-to-gable loft conversions: when permitted development stops
A hip-to-gable uses more of the Class B volume allowance than most people expect, and the house type that suits the form is often the one with the smaller limit.
Hip to gable permitted development is allowed under Class B of Part 1 of Schedule 2 to the GPDO 2015, and it stops at four points: the volume allowance, the roof slope that fronts a highway, the height of the existing ridge, and article 2(3) land. Miss any of them and the conversion needs planning permission, however ordinary it looks from the street.
The volume allowance is where most schemes come unstuck, because a hip-to-gable eats a surprising amount of it before the dormer is even drawn.
The volume allowance and what counts
Class B permits an addition or alteration to the roof provided the cubic content of the resulting roof space does not exceed the cubic content of the original roof space by more than 40 cubic metres for a terrace house or 50 cubic metres in any other case.
Three points about that measurement.
It is the cubic content of the roof space, not the floor area of the new room, and it is measured externally. So the volume includes the thickness of the roof construction, which is why a builder's estimate based on internal room size runs low.
It is cumulative. Any earlier roof enlargement counts against the allowance, including one carried out by a previous owner decades ago. If a rear dormer was added in 1998 and used 18 cubic metres, you have the balance and not the full figure.
And the baseline is the original roof space, meaning the roof as the house was built, or as it stood on 1 July 1948 if the house predates that. A house that has already been extended at the rear is measured against its original roof, not its current one.
The 40 cubic metre trap for end-terrace houses
Part 1 defines a terrace house as a dwellinghouse in a row of three or more buildings used as dwellinghouses, where it shares a party wall with, or has a main wall adjoining the main wall of, the building on either side, or on one side if it stands at the end of the row.
Read that last clause carefully. An end-of-terrace house is a terrace house for these purposes, so the allowance is 40 cubic metres and not 50.
That matters because the end-of-terrace house is exactly the one with a hip to convert. A mid-terrace house usually has no hip at all. So the properties where a hip-to-gable is physically possible are often the ones with the smaller allowance, and a full-width hip-to-gable on a modest end terrace can consume most of it.
Semi-detached and detached houses get 50 cubic metres. Chalet bungalows, which frequently have generous hips, are usually in the 50 cubic metre category and still run out of allowance because the hip volume is large relative to the original roof.
Roof slopes fronting a highway
Class B does not permit any part of the enlargement to extend beyond the plane of an existing roof slope that forms the principal elevation of the house and fronts a highway.
For a typical suburban semi the hip sits on the flank, the principal elevation faces the road, and a hip-to-gable on the side is unaffected. For a corner plot the geometry changes. Where the hipped slope itself faces the highway and forms part of the principal elevation, the enlargement is outside Class B and a planning application follows.
There is a separate height limit. No part of the enlargement may exceed the height of the highest part of the existing roof, so a gable raised above the existing ridge line is not permitted development. Nor is a balcony, veranda or raised platform formed within the roof, which rules out the Juliet arrangement some designers propose at the new gable end.
The 20 centimetre eaves rule and its exception
Class B carries a condition that the edge of the enlargement closest to the eaves of the original roof must, so far as practicable, be set back at least 20 centimetres from the eaves, measured along the roof slope.
The exception is the part practitioners forget. That condition does not apply to a hip-to-gable enlargement, nor to an enlargement joining the original roof to that of a neighbouring house. A hip-to-gable may therefore be built out to the eaves line, which is what makes the form work at all, since a gable set back 200mm from the eaves would look absurd.
The set-back does still apply to a rear dormer built at the same time, so on a combined scheme the dormer needs the 200mm and the gable does not. Two other Class B conditions apply throughout: materials must be similar in appearance to the existing house, and any side-facing window must be obscure-glazed and non-opening below 1.7 metres above the floor.
Conservation areas and article 4 directions
Class B is not available at all on article 2(3) land, which covers conservation areas, National Parks, National Landscapes, the Broads and World Heritage Sites. A hip-to-gable in a conservation area needs planning permission, and it will be assessed against the duty in section 72 of the Planning (Listed Buildings and Conservation Areas) Act 1990 to pay special attention to preserving or enhancing the character or appearance of the area.
In practice hip-to-gable conversions are refused in many conservation areas, because the hipped roofline is often part of what the appraisal identifies as characteristic. A Heritage Statement has to engage with that rather than assert that the change is minor, and where the roof form is a stated characteristic the honest advice is usually to design a scheme that keeps the hip.
Outside conservation areas, an article 4 direction can remove Class B rights for a defined area. Check the council's article 4 register for the address before assuming the rights exist.
Party wall and structural consequences
Removing a hip removes triangulation from the roof structure. The new gable wall usually needs to be built off a supported line, which means steel at ceiling level and often a new padstone bearing into the party wall.
That engages the Party Wall etc. Act 1996. Cutting into a party wall, inserting beams and building on the line of junction all require notice to the adjoining owner, and dissent leads to an award. Permitted development status does not remove any of it, and neither does a planning permission.
On an end-of-terrace house there is a further point. Where the gable is built up on the flank wall rather than the party wall, the foundation below may not have been designed for the additional load, and a trial hole is worth digging before the design is fixed.
Combining a hip-to-gable with a rear dormer
The usual scheme raises the hip to a gable and adds a rear dormer, and both count against the same 40 or 50 cubic metre allowance.
Work the volumes in this order:
- Calculate the original roof space volume from the original geometry, not the current one.
- Deduct any previous roof enlargement, including work by previous owners.
- Calculate the hip-to-gable volume, which is the wedge added at the hipped end.
- Calculate the proposed dormer volume, measured externally.
- Compare the total against 40 or 50 cubic metres depending on whether the house is a terrace house as defined.
Illustrative example. An end-terrace 1930s house has a hip on one flank. The hip-to-gable wedge measures around 24 cubic metres, leaving 16 against the 40 cubic metre limit. A full-width box dormer would take roughly 27, so the scheme goes over. Reducing the dormer to a narrower box set in from both edges and down from the ridge brings the total under, and the smaller dormer is a better piece of design in any case. The figures are illustrative.
Where the total exceeds the allowance, a full planning application is needed, and a short Planning Statement setting out the volumes, the roof form and the effect on the street is worth including. If the scheme also involves work at the back, check the rear extension depth limits at the same time, since the two sets of rights are measured separately.
A Lawful Development Certificate is the only formal way to confirm that a loft conversion is lawful without permission, and for a hip-to-gable close to the volume limit it is money well spent.
Enter your postcode on PlanScope to see which documents a loft conversion application is likely to need.
PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval. This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it.
Frequently asked questions
How much volume does Class B allow for a loft conversion?
Forty cubic metres for a terrace house and 50 cubic metres in any other case, measured as the increase in the cubic content of the roof space over the original roof. It is cumulative, so earlier roof enlargements by any owner count against the allowance.
Does an end-of-terrace house get 40 or 50 cubic metres?
Forty. Part 1 defines a terrace house to include a dwelling at the end of a row of three or more, adjoining a neighbouring main wall on one side. That is the house type most likely to have a hip to convert, so the smaller allowance applies exactly where it hurts.
Does the 20 centimetre eaves set-back apply to a hip-to-gable?
No. The Class B condition requiring a 200mm set-back from the eaves is expressly disapplied for hip-to-gable enlargements and for enlargements joining the original roof to a neighbouring roof. It still applies to a rear dormer built as part of the same scheme.
Can I do a hip-to-gable in a conservation area?
Not as permitted development. Class B is unavailable on article 2(3) land, which includes conservation areas, so a planning application is needed and the section 72 duty applies. Where the conservation area appraisal identifies hipped roofs as characteristic, the application is often refused.
Does removing a hip need party wall notices?
Usually yes. The new gable normally requires steel bearing into the party wall, and cutting into or building on the line of junction engages the Party Wall etc. Act 1996. Permitted development status does not remove that requirement, and neither does a planning permission.
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.