Hip-to-Gable Loft Conversions Explained: Cost, Planning & What to Expect (West London Guide)
If you live in a 1930s semi in Hayes, Ruislip, Hounslow, Greenford or Harrow, the loft conversion you almost certainly need is a hip-to-gable. It is the conversion GSM Builders Ltd builds more than any other, because it is the only way to turn the cramped hipped roof of an inter-war semi into a full-size bedroom and bathroom. This guide explains what it is, what it costs in 2026, the planning rules, and what to expect on site.
What is a hip-to-gable loft conversion?
A hipped roof slopes on three sides – front, back and one side – which leaves very little usable floor space with full head height. A hip-to-gable conversion builds the sloping side up into a vertical gable wall, so the roof becomes a simple two-sided pitch. That alone adds a wedge of space; combined with a rear dormer across the back, it creates a rectangular room of 25–35 m² – enough for a double bedroom, an en-suite and eaves storage.
Hip-to-gable loft conversion cost in 2026
| Specification | West London cost (ex VAT) | On site |
|---|---|---|
| Hip-to-gable + rear dormer, bedroom only | £55,000 – £72,000 | 8–9 weeks |
| Hip-to-gable + rear dormer, bedroom + en-suite | £62,000 – £85,000 | 9–10 weeks |
| Double hip-to-gable (detached house), two rooms + bathroom | £80,000 – £110,000 | 10–12 weeks |
| Hip-to-gable with Velux only (no dormer) | £48,000 – £60,000 | 7–8 weeks |
Prices include scaffold, structural steel, the new gable wall, dormer, roof, windows, staircase, insulation, electrics, plumbing, plastering, bathroom fit-out and decoration. They exclude VAT and professional fees. See the full loft conversion cost guide or use the calculator.
Why 1930s semis in West London suit hip-to-gable
Hillingdon, Hounslow, Ealing’s outer suburbs and Harrow were largely built between 1925 and 1939 as “Metroland”: rows of hipped-roof semis with a side passage, bay window and a generous ridge height, usually 2.4–2.8 m from the ceiling joists. That ridge height is what makes the conversion work – you need at least 2.2 m after the new floor goes in. The gable wall is built on the flank wall of the house, which on a semi is the outside wall, so there is no neighbour to share it with. The result is the largest loft you can get from this house type without raising the ridge.
Hip-to-gable vs other loft types
| Type | Best for | Space gained | Cost |
|---|---|---|---|
| Velux / rooflight | Roofs with 2.3 m+ head height, tight budgets | Least – keeps the sloping ceilings | £35k–£48k |
| Rear dormer | Terraces with a gable end already | Good – full-height rear half | £48k–£70k |
| Hip-to-gable + dormer | 1930s semis and hipped detached houses | Most – full-width rectangular room | £58k–£85k |
| L-shaped dormer | Victorian terraces with a rear outrigger | Very large – two rooms possible | £65k–£88k |
| Mansard | Conservation areas, flat-front terraces | Largest but needs planning | £75k–£110k+ |
Planning permission for a hip-to-gable loft
On a house (not a flat), a hip-to-gable with rear dormer is normally permitted development if the added roof volume is under 50 m³ for a semi or detached house, the dormer sits at least 20 cm above the eaves and below the ridge, no part faces the road, and materials match. Most 1930s semis come in at 40–48 m³, so it fits – but it is tight, and a previous rear extension with a raised roof can count against the allowance. Where it does not fit, a full application (£258, eight weeks) is needed.
You will need planning permission in conservation areas with Article 4 directions – Ruislip Village, Eastcote Village, Ickenham Village, Old Uxbridge, Harrow on the Hill, Pinner High Street, Ealing Common and others – and on flats. We always recommend a Lawful Development Certificate (£129) so you have proof for buyers. Read our full planning permission guide.
Building Regulations: what Building Control checks
- Structure – steel beams spanning the party wall to the new gable, new floor joists, and the dormer frame, all from an engineer’s calculations
- Fire safety – a protected stair enclosure with fire doors to every room off it, mains-linked smoke alarms on each floor, and an escape window in the loft
- Insulation – gable wall, dormer walls and roof to current U-values
- Stairs – maximum 42° pitch, 2 m headroom (1.9 m allowed at the edge of the stair)
- Bathroom – drainage falls, ventilation and an accessible soil connection
- Sound – insulation between the loft floor and the bedrooms below
The Party Wall Act
The steel beams that carry the new floor and the dormer normally bear on the party wall shared with your neighbour, so a hip-to-gable on a semi always triggers the Party Wall Act. Notice must be served two months before work starts. Most neighbours consent in writing, especially if you show them the drawings first; if not, a surveyor’s award costs £900–£2,000. GSM Builders Ltd serves the notices for you.
What happens on site, week by week
- Week 1: scaffold, roof opened at the hip, steel beams craned or hand-carried in and set on padstones
- Weeks 2–3: gable wall built up in blockwork and faced in matching brick or tile; new floor joists fitted; dormer framed and roofed
- Week 4: roof made watertight; dormer clad; windows in
- Weeks 5–6: staircase cut in; first-fix electrics and plumbing; insulation
- Weeks 7–8: plasterboard and plaster; second fix; bathroom fitted
- Weeks 9–10: decoration, flooring, Building Control final inspection, snagging
You stay at home throughout. The house is only opened to the new floor when the staircase goes in around week five, and even then the loft is sealed off with a temporary door.
Common problems – and how we deal with them
- Low ridge height: under 2.2 m we can sometimes lower the ceilings below (if they are tall) or use a Velux-only design; raising the ridge needs planning.
- Trussed roofs on post-1970s houses: trusses are removed and replaced with steel and timber – we do this regularly.
- Water tank and boiler in the loft: moved to a cupboard or replaced with a combi as part of the job.
- Chimney breast: can be removed with a gallows bracket or steel, subject to Building Control and the Party Wall Act.
- Heathrow noise in Hayes, Harlington, Hounslow and Cranford: acoustic glazing and upgraded insulation specified as standard.
Hip-to-gable lofts with GSM Builders Ltd
We build hip-to-gable conversions across Hayes, Hillingdon, Uxbridge, Ruislip, Hounslow, Greenford and Harrow – drawings, structural calculations, Lawful Development Certificate, Party Wall notices, build and Building Control sign-off under one contract, with a 12-month warranty. Free surveys anywhere in West London.
Hip-to-gable loft conversion FAQs
How much does a hip-to-gable loft conversion cost?
In West London a hip-to-gable loft conversion with rear dormer costs £55,000–£72,000 for a bedroom, or £62,000–£85,000 with an en-suite, excluding VAT. Double hip-to-gables on detached houses are £80,000–£110,000. GSM Builders Ltd surveys and quotes free.
Do you need planning permission for a hip-to-gable loft conversion?
Usually not on a house: it is permitted development if the added roof volume is under 50 m³ on a semi or detached house, the dormer is at the rear and materials match. Conservation areas with Article 4 directions and all flats need planning permission. A Lawful Development Certificate is recommended.
How much space does a hip-to-gable add?
Typically a 25–35 m² room on a 1930s semi – enough for a double bedroom, en-suite and eaves storage – compared with 15–20 m² of usable space from a Velux conversion on the same roof.
How long does a hip-to-gable loft take?
Eight to ten weeks on site, plus two to four weeks beforehand for drawings, structural calculations and approvals.
Is a hip-to-gable conversion suitable for a detached house?
Yes – on a detached house both hips can be converted (a double hip-to-gable), often creating two bedrooms and a bathroom.
Does a hip-to-gable loft need a Party Wall agreement?
On a semi, yes – the steel beams bear on the party wall, so notice must be served to the neighbour two months before work starts. GSM Builders Ltd handles this.
