The answer for a hipped roof, and the reason 1930s semis in Whitchurch and Llanishen convert so well.
Arrange a site visitA hipped roof slopes on three or four sides instead of ending in a flat vertical wall. It is the standard roof on the interwar and 1930s semis that make up much of Whitchurch, Llanishen, Heath, Rhiwbina and Birchgrove. It looks good from the street and it is a problem in the loft, because the sloping end wall cuts the usable floor area down to very little — often a narrow strip that will not take a bed, let alone a staircase.
A hip-to-gable conversion rebuilds that sloping end as a vertical gable wall, in masonry, taking the roof out to the full width of the house. The roof is then extended over the new wall. What you gain is not a small improvement: it is usually the difference between an unusable loft and a full room.
On its own, a hip-to-gable gives you width but the roof still slopes front and back. In practice it is normally combined with a rear dormer, which supplies the headroom. The two together are what turn a 1930s semi loft into a double bedroom with an en-suite. Priced separately they look like two jobs; built together they share scaffold, structure and making good, which is why the combined figure is less than the sum of the parts.
Hip-to-gable work uses more of the volume allowance than a dormer alone, because you are adding the whole triangular section of roof as well as the dormer. The allowance commonly cited is around 50 cubic metres for a semi-detached or detached house. Many hip-to-gable conversions do fall within permitted development, but the combination with a large dormer is exactly the case where the volume runs out, so it is worth checking before the design is fixed. Conservation area status removes the permitted development route entirely.
Published UK guides put hip-to-gable conversions at roughly £45,000 to £65,000 in 2026, reflecting the masonry and structural work a dormer alone does not involve. Those are national averages rather than our quotes; our cost page sets out what moves the number. On site, eight to twelve weeks is a more realistic expectation than the six to eight a simple dormer takes, because the gable has to be built and made weathertight before the internal work can progress.
A short site visit tells us the roof type, the head height and where the stairs can land — which is most of what decides the design and the price. We will tell you if a conversion is not the right answer for your house.
Arrange a site visitAll loft conversions