Hip to gable conversions and the roof volume budget

Turning a hipped roof end into a gable is permitted development under Class B of the General Permitted Development Order, provided the whole roof enlargement stays inside a volume allowance of 40 cubic metres for a terrace house or 50 cubic metres for anything else. The hip to gable loft conversion rules that catch people out are not really about the gable. They are about what is left in the budget once the gable is built, because the rear dormer that makes the loft usable is measured against the same allowance.

Get the order of operations wrong and you build half a conversion lawfully and the other half without permission.

What the gable adds to the roof volume

A hipped end slopes back on three sides. Taking the hip off and carrying the end wall up to the ridge replaces a wedge of open air with enclosed roof space, and every cubic metre of it counts.

The allowance is cumulative and it is measured against the original roof space, which means the roof as first built or as it stood on 1 July 1948, whichever is later. A dormer added by a previous owner in 1998 has already spent part of the budget even though you did not build it. Land Registry plans will not tell you this. Old aerial photography and the council's own planning history usually will.

Class B applies to houses. Flats and maisonettes have no permitted development rights for roof alterations at all, and neither does a dwelling created through a Part 3 prior approval, such as an office converted under Class MA or an agricultural building under Class Q.

Measuring the added volume correctly

Volume is measured externally, including the thickness of the walls and the roof covering. Measuring to the inside face of the plasterboard will flatter the figure by a couple of cubic metres, which is exactly the margin people lose arguments over.

On a roof pitched equally on all sides, the hip runs back from the end wall by roughly half the span, and the space you gain is the difference between a triangular prism over that length and the pyramid the hip currently encloses. That difference comes out at about one sixth of the span multiplied by the roof height above eaves level multiplied by the hip run.

Illustrative example. A semi-detached house with a 7.5 m span, a roof standing 3.1 m above the eaves and a hip running 3.75 m back from the end wall. One sixth of 7.5 x 3.1 x 3.75 is roughly 14.5 m3. Against a 50 m3 allowance that leaves about 35 m3 for everything else. On an end of terrace house the allowance is 40 m3, so the same gable leaves about 25 m3. These numbers are illustrative only. Measure the roof you actually have, and draw the calculation on a section.

Adding a rear dormer to the same budget

This is where schemes fall over. A full width box dormer is a simple rectangular solid, so a dormer 5 m wide, 3 m deep and 2.4 m high adds around 36 m3. Add the 14.5 m3 gable from the example above and you are at 50.5 m3, over the limit for a semi and comfortably over it for an end of terrace.

Two conditions apply to the dormer that do not apply to the gable. The first is the 20 cm set back: any roof enlargement must sit at least 20 cm from the eaves of the original roof, measured along the slope, and Class B exempts a hip to gable enlargement from that requirement but not a dormer. The second is glazing. Any window in a side elevation of the enlargement must be obscure-glazed and, if it is below 1.7 m above the floor of the room it serves, non-opening.

Trimming a dormer back by 300 mm in depth usually costs less than a householder application and the eight week wait that comes with it. Price both before the drawings are finalised.

Where Class B does not apply at all

Class B is withdrawn completely on article 2(3) land. That covers conservation areas, National Landscapes (the designation formerly called Areas of Outstanding Natural Beauty), National Parks, the Broads and World Heritage Sites. There is no reduced allowance in those areas. There is no allowance.

An Article 4 direction can remove the right elsewhere, and plenty of councils have made them over inter-war suburbs with consistent roofscapes. Check the constraints layer on the council's map before you assume.

Three further exclusions sit in Class B itself. Nothing may rise above the highest part of the existing roof. Nothing may extend beyond the plane of the roof slope that forms the principal elevation where that elevation fronts a highway, which is what stops front dormers and which needs care on a corner plot where the council may treat the flank as a principal elevation. And no veranda, balcony or raised platform is permitted, so a roof terrace cut into the slope always needs an application. If the house is listed, none of this removes the need for listed building consent, and a Heritage Statement will be expected with it.

The principal elevation question comes up often on corner sites, and the same reasoning that applies to a rear extension on a corner plot applies to a hip that faces the side road.

The party wall you are building on

On a semi or an end of terrace, the new gable is very often built off or into the party wall. That triggers the Party Wall etc. Act 1996, which is a civil regime with nothing to do with planning. Section 2 works to a party wall need two months notice to the adjoining owner. Excavation near a neighbour's foundations under section 6 needs one month.

The council will not check any of this and will not mention it. The neighbour's surveyor will.

Head height and what the gable buys you

A gable does not raise the ridge, so it does not create head height. What it creates is usable floor width at the end of the loft, because the sloping hip that was cutting the room off at an angle has gone. On a typical inter-war semi that is the difference between a bedroom with a cupboard and a bedroom with a shower room.

Building Regulations in England set no minimum ceiling height for a habitable room, which surprises people. The binding dimensions are in Approved Document K for headroom on the new stair and Approved Document B for escape, fire doors and the protected route. Fitting sanitary accommodation into the remaining space has its own constraints, covered in our note on putting a shower room into a loft conversion. Where the roof is too shallow for any of this to work, the alternative is usually adding a first floor within the roof of a chalet bungalow, which is a different permitted development question entirely.

Proving it was lawful afterwards

Permitted development is a right, not a permission, so nothing is issued at the end of it. The only formal way to confirm that works are lawful is a lawful development certificate: section 192 of the Town and Country Planning Act 1990 for proposed works, section 191 for works already done. Apply for the proposed version before you start. The fee is a proportion of the equivalent planning application fee and is indexed, so check the current fee schedule on the Planning Portal.

A certificate application needs existing and proposed sections at a proper scale with the volume calculation set out on the drawing, not buried in a covering letter. Officers check the arithmetic. Make it easy to check and the determination is quick.

Buyers' solicitors ask for the certificate five or ten years later, and its absence is a routine reason for a sale to stall. That alone justifies the fee.

Enter your postcode on PlanScope to see which supporting documents a loft conversion application on your site 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.