Hip to gable loft conversions: cost, planning and what to know

A hip to gable loft conversion rebuilds the sloping end of a hipped roof as a vertical wall, so the loft runs the full width of the house instead of shrinking under the hip. It costs £54,000 to £78,000 including VAT in 2026. We cover which houses can take one, the full budget, the steel it needs, the planning and building regulations rules in each UK nation, and how it compares with a dormer.

What a hip to gable loft conversion is

A hipped roof slopes down on all four sides to meet the walls. The end slope eats into the loft, so the space under it is too low to stand in and too awkward to furnish.

A hip to gable conversion builds the end wall of the house straight up to the ridge and reframes the roof against it, so the sloping end becomes a vertical gable. The ridge stays at its existing height: the conversion squares off the end rather than raising the roof.

A hip to gable is often built with a rear dormer. The gable adds width across the loft and the dormer adds standing height at the back, so the two are designed as one job.

Which houses suit a hip to gable

Only a house with a hip to start with. In practice that means:

• Semi-detached houses with a hipped outer end, the most common candidate by far. Many are 1930s and 1950s semis.
• End-of-terrace houses whose outer end is hipped.
• Detached houses, where both ends can be squared off in a double hip to gable, within one 50 cubic metre allowance.

On a pair of semis with matching hips, squaring off one half changes how the pair looks from the street. That has no bearing on permitted development, which tests size and position rather than appearance.

It is the wrong route when:

• The house is mid-terrace. There is no hip to square off, so the options are a rear dormer, rooflights or a mansard with planning permission.
• The house is in a conservation area, where permitted development for a roof enlargement does not apply and you need a planning application.
• The home is a flat, which has no permitted development rights for a roof.
• The loft is under about 2.2 metres from ceiling joist to ridge. A gable adds width, not height, so a low loft needs a mansard or a piggyback.

Hip to gable loft conversion cost

For a hip to gable, loft conversion prices run £54,000 to £78,000 including VAT at 20% in 2026, above a dormer and below a mansard. No official body publishes loft conversion prices, so these are market rates.

That price covers the conversion and the fit-out of the room. The staircase, the steel for the new floor, the fees and any en suite usually come on top, so we would budget from the card below rather than the headline figure.

What pushes a hip to gable quote up

• A trussed roof: most houses from the mid-1970s on have trussed rafters, which need new steel at wall plate level and a new floor hung from it, at £5,000 to £15,000 more.
• A rear dormer as well: the dormer brings its own box, roof and steel, and the pair has to fit one volume allowance.
• Both ends of a detached house: two gable walls to build and two hips to reframe.
• Weak foundations: the new gable puts more weight on the side wall, and if its foundations cannot take it they may need underpinning.

Hip to gable costs in London

A hip to gable in London costs £63,000 to £91,000 including VAT, because building prices in inner London run about 17% above the UK average. Hipped semis are a common house type in outer boroughs such as Barnet, Ealing, Bromley and Croydon.

Do you need planning permission for a hip to gable?

In England, a hip to gable usually needs no planning application, because it counts as permitted development under Class B, the same class as a dormer. The government's technical guidance sets the limits:

• No more than 40 cubic metres of added roof space on a terraced house, end of terrace included, or 50 on a semi-detached or detached house, counting any earlier roof enlargement.
• Nothing higher than the highest part of the existing roof, so the gable stops at the ridge.
• Nothing beyond the roof slope on the front of the house where it faces a road.
• Materials similar in appearance to the house.
• Any window in a side wall or side roof slope obscure-glazed, and fixed unless the opening part is more than 1.7 metres above the floor. On a semi, the new gable is a side wall.
• No verandas, balconies or raised platforms.
• Not a flat, not on designated land such as a conservation area, National Park or AONB, and not where an Article 4 direction or a planning condition has removed the rights.

One rule that applies to a dormer does not apply here. A roof enlargement normally has to sit at least 20 centimetres back from the original eaves, but the guidance exempts a hip to gable, so the new gable rises straight off the side wall.

How much of the volume allowance a hip to gable uses

A hip to gable fills in the wedge of roof space under the hipped end, and that counts toward the allowance before any dormer is added. Where the hip runs at the same pitch as the main roof, the wedge comes to roughly the house's depth, squared, times the height from the ceiling joists to the ridge, divided by 12. A rear dormer adds its width times its front wall height times its depth, halved.

A typical semi fits a hip to gable and a full-width dormer. A deep semi with a tall roof, or a detached house squared off at both ends, can run out, and anything a previous owner added counts too. Your designer works out the exact figure from a measured survey.

Where the design fits the limits, a £274 Lawful Development Certificate confirms it in writing, and a buyer's solicitor will want to see it later. Where it does not, planning permission for the loft conversion costs £548 in England, with a decision due about 8 weeks from validation.

Party wall notices for a hip to gable

In England and Wales, a hip to gable on a semi or an end of terrace usually means work on the party wall, such as cutting in the ends of the steel floor beams. Cutting into a party wall to take the bearing of a beam is a right under the Party Wall etc. Act 1996, but it comes with notice.

You serve a party structure notice at least 2 months before the work, and the neighbour has 14 days to reply. Written consent costs nothing. No reply, or a dissent, means a surveyor's award at £1,200 to £3,000. A detached house usually has no party wall to notify.

Building regulations for a hip to gable

Every hip to gable needs building regulations approval, including one built under permitted development. A full plans application to the council or a registered building control approver gets a decision within 5 weeks, or 2 months if you agree to more time. The checks that matter most are the structure of the new gable and roof, the stair, the fire strategy and the insulation.

Fire safety changes once the new floor sits more than 4.5 metres above ground, where escaping through a window is no longer acceptable. On a typical two-storey house the stairway becomes the escape route, protected by fire-resisting doors and sometimes new partitions, with mains-powered, interlinked smoke alarms in the stairway at each level.

The new room needs a proper stair, not a retractable ladder. Approved Document K asks for 2 metres of headroom over the stair, relaxed in a loft conversion to 1.9 metres at the centre and 1.8 metres at the side.

Building control issues a completion certificate, usually within 8 weeks of the work finishing. A buyer's solicitor will ask for it, and a conversion without one can lower the price of the house.

What structural work a hip to gable needs

Taking the hip away removes the support it gave the ridge and the end of the roof. Its loads have to go somewhere else: into a new ridge beam, extra purlin support where needed, and the new gable wall.

The gable is built off the existing side wall, in blockwork or as a timber frame, then finished in brick or render to match. It carries roof loads at its top and has to resist wind pushing on it from the side. The wall below and its foundations then carry more weight, so building control can ask for the foundations to be checked and, where they fall short, underpinned.

Inside, the building regulations for new floors in a loft assume the existing ceiling joists will not carry a habitable room. New, usually deeper joists go in between them, sitting on the walls where those are strong enough and on steel or timber beams where they are not, and those beams need fire protection.

Hip to gable on a 1930s semi

A 1930s semi usually has a cut roof, built on site from separate rafters and purlins, as most houses from before about 1965 do, and a cut roof is the easier kind to alter. Most houses from the mid-1970s on have trussed rafters, which cannot simply be cut away and need new steel at wall plate level.

Every loft conversion needs a structural engineer's calculation pack for building control, and it usually takes 5 to 7 working days. For a hip to gable the engineer charges £600 to £1,100, against £450 to £700 for a rear dormer.

How long a hip to gable loft conversion takes

A hip to gable takes 10 to 14 weeks on site, after 2 to 3 months of drawings, structural calculations and approvals. Within a larger project it comes early in the order of a full renovation, because structural repairs and roofing go before services, insulation and final finishes.

Before the build, the work runs in this order:

1. A measured survey of the house and the loft.
2. Design drawings: floor plans, elevations and sections, with the added volume worked out.
3. A permitted development check, then a Lawful Development Certificate or a planning application.
4. Building regulations drawings and the structural engineer's calculations.
5. Submission to building control, and the party structure notice to the neighbour.

Hip to gable or dormer: which to choose

A dormer works on most pitched roofs. A hip to gable only works where there is a hip, and on a hipped semi the two are often built together, because the hip leaves the rear slope too narrow for a full-width dormer on its own.

On its own, a hip to gable costs £6,000 to £12,000 more than a dormer and takes about 4 weeks longer on site. Choose it where the roof is hipped and the room needs the width. Where the roof already ends in a gable, a rear dormer does the job alone.

Hip to gable rules in Wales, Scotland and Northern Ireland

The limits above are England's. The other three nations set their own roof rules and run their own building control.

The Welsh Government's loft conversion rules exempt a hip to gable from the 20 centimetre set-back, as England does. They also want side windows within 10.5 metres of a boundary obscure-glazed or non-opening, and rule out UPVC other than, possibly, for windows. The planning enforcement limit in Wales is 4 years, against 10 in England.

Scotland's limits, from the Scottish Government's householder circular, turn on width and distance rather than volume. A hip to gable removes a whole roof slope, so check with the council how those limits apply to yours before you design around permitted development. A building warrant from the council may be required as well.

Northern Ireland's roof limits come from its 2015 permitted development order, and its 0.5 metre gap to the eaves and verge raises the same question, so ask the council first there too. The Party Wall etc. Act 1996 does not apply in Scotland or Northern Ireland.