Hip to Gable Loft Conversion London

A hip to gable loft conversion rebuilds the sloping end of a hipped roof as a vertical gable wall, taking the ridge out to the full width of the house. It exists for one reason: a hipped roof wastes the space at the end of the loft, and building the hip up to a gable is the only way of getting it back.

It is a suburban London project rather than an inner London one. The houses it suits are the interwar semis that fill Barnet, Enfield, Harrow, Ealing and the outer boroughs, and the end terraces that have a hip on the exposed flank. A mid terrace has no hip and cannot have this conversion, which is a useful two second test.

What a hipped roof costs you in usable space

On a gable ended roof the ridge runs the full depth of the house and the loft is a long triangular volume with usable headroom down the middle. On a hipped roof the end slopes inwards as well, so the ridge stops short and the volume tapers away to nothing at that end. The floor area with the 1.9 metres or more of headroom you actually need is a fraction of the footprint, and the shape it leaves is awkward: a narrow strip with sloping ceilings closing in from three sides.

Building the hip up to a vertical gable does three things at once. It takes the ridge out to the flank wall, which lengthens the usable volume. It replaces sloping ceiling with full height wall on that side, which is where a wardrobe or a bed head can actually go. And it produces a roof shape that a rear dormer can then be built into, which is why hip to gable and dormer are almost always done as one project rather than two.

The head height check comes first, exactly as on any loft. Measure from the top of the ceiling joists to the underside of the ridge: below about 2.2 metres a conversion gets difficult, and 2.4 metres or more leaves room for the new floor structure and insulation. On a hipped roof, measure at the ridge rather than anywhere near the hip, because the hip end will read low and tell you nothing useful.

What the work involves structurally

This is more structural than a rear dormer, because you are removing part of a roof that is holding itself up and building masonry where there was none.

• The hip end of the roof is stripped and removed, with the remaining roof supported temporarily while it is open
• A new gable wall is built up in blockwork or brickwork off the existing flank wall below, which means that flank wall and its foundation have to be capable of taking the additional load. The engineer checks this rather than assuming it
• New ridge, purlin and rafter arrangement to carry the roof out to the new gable, all sized by the structural engineer
• New floor joists spanning onto the load bearing walls or onto steel beams, designed by the engineer and never sat on the existing ceiling joists
• A rear dormer built into the enlarged roof in the same contract, since the gable on its own rarely produces enough usable room to justify the work
• Matching the external brickwork or render, which on a 1930s semi is the difference between a conversion that reads as original and one that reads as an addition
• Full building regulations package: protected escape stair, FD30 doors, interlinked mains alarms, Part K compliant staircase, Part L insulation and proper ventilation

GS Renovation is the contractor. The gable, the roof structure and the new floor are designed by an independent structural engineer we appoint on your behalf, we build to those calculations, and building control inspects and certifies. We do not produce structural calculations.

Planning permission and the semi-detached problem

A hip to gable can fall within permitted development, and frequently does on an interwar semi. The volume allowance is the governing number, commonly cited as 50 cubic metres of additional roof space on a semi-detached or detached house and 40 cubic metres on a terrace, cumulative with any earlier roof enlargement. That matters here more than on any other loft type, because a hip to gable uses a substantial part of the allowance before the dormer is even considered, and the two together often exceed it. Where they do, a householder planning application follows.

The conditions apply as they do to any roof enlargement: nothing beyond the roof plane on the elevation facing a highway, nothing above the existing ridge height, similar materials, obscure glazed side windows. The 200 millimetre eaves setback that applies to a dormer does not apply to the gable itself, which is one of the few concessions in Class B. Rights are removed altogether in conservation areas, on listed buildings and under Article 4, and flats have none.

The other consideration on a semi is your neighbour's roof. A pair of interwar semis was designed as one symmetrical building with a hip at each end, and putting a gable on one half changes that. Some councils are entirely relaxed about it, others assess the effect on the pair and on the street scene, and a handful of streets have consistent refusals. It is worth checking what has been approved nearby before drawings are commissioned. If both halves of the pair convert, the symmetry returns, which is why these often happen in twos along a street.

Party wall on a semi-detached house

On a semi, a hip to gable engages the Party Wall etc. Act 1996 almost by definition. The new gable is built off the flank wall, the new floor beams bear into or on the party wall, and the roof is cut back and made good against it. Notice must be served on the adjoining owner before work starts, and the duty to serve is yours as the building owner rather than ours as the contractor. We identify the notices at quotation stage and build the statutory response periods into the programme. Where a neighbour dissents, surveyors are appointed and the award governs how the work is carried out. We build to the award, and we are not party wall surveyors.

What a hip to gable conversion costs in London

Scope	What it covers	Typical London cost
Hip to gable plus rear dormer	The usual complete project: gable built up, roof extended, dormer formed, one or two rooms created	£55,000 to £80,000
Rear dormer only, for comparison	Where the roof is already gable ended and no hip work is needed	£45,000 to £65,000
Rooflight conversion, for comparison	Existing roof shape retained, no structural roof alteration	£30,000 to £45,000
Mansard, for comparison	Roof rebuilt with steep front and rear slopes for maximum volume	£70,000 to £100,000+
En suite added	Shower room formed within the new loft	£8,000 to £15,000
Party wall surveyors	Per adjoining owner with a single agreed surveyor	£900 to £1,200
Professional fees	Structural engineer, designer, planning or lawful development certificate, building control	10 to 15 per cent of build cost

Indicative London ranges, 2026. Trade and project costs from MyBuilder and Checkatrade; per square metre build rates from Architecture for London and Resi.

The comparison that matters on a 1930s semi is not between loft types, it is between the loft and a rear extension, because these houses usually have the garden depth for both. Our extension versus loft conversion guide sets the two against each other, and the London loft conversion cost guide breaks the loft figures down by type.

Where we do most of this work

Hip to gable work follows the interwar housing, which means the outer boroughs. Barnet, Enfield, Harrow, Ealing and the suburban parts of north and west London are where the roof form is most common, and where we do most of it. The area pages below cover loft conversions locally, and the two other roof forms are on their own pages: a dormer where the roof is already gable ended, and a mansard where the ridge height will not give you a room any other way.