Semi-detached and detached houses built between the wars very often have a hipped roof — the end slopes inward instead of running up to a gable. That hip steals usable volume from exactly the part of the loft where a landing or bathroom would go.
Hip-to-gable work removes the sloping hip and builds the end wall up vertically, in brick or timber frame, to match the existing gable line. A rear dormer is a box built out from the rear roof slope, independent of what the hip is doing.
They solve different problems, which is why surveyors so often recommend combining them rather than picking one.
What each one actually adds
Hip-to-gable adds width along the whole depth of the roof, not just at one end. On a semi it typically brings the loft up to something close to the full footprint of the floor below, because the sloping hip was previously eating into that end.
A rear dormer adds height and a flat ceiling at the back, which is where a stair usually needs to land and where a bathroom needs standing headroom under Building Regulations. It does nothing for the sides of the roof.
Planning and permitted development
A rear dormer on a semi is very often permitted development, subject to volume limits and side boundary setbacks. Hip-to-gable is also frequently permitted development on its own, but combining the two, or building on a corner plot, can tip the scheme over the threshold and require full planning permission.
Article 4 directions and conservation area designations remove permitted development rights in some parts of Kent and East Sussex, so this always needs checking against the specific address before design work starts, not after.
Structure and sequencing
Hip-to-gable is more disruptive to the existing roof structure because the hip rafters and hip end are being removed and rebuilt as a new wall, usually needing new steel or timber support at eaves level. A rear dormer sits within the existing roof slope and generally disturbs less of the original structure.
Where both are being done, hip-to-gable is normally completed first so the new gable wall gives the dormer something solid to tie into at the corner.
Cost and what drives it
Hip-to-gable involves more masonry or framing, more roof re-covering, and usually a steel beam at eaves level to carry the new gable — all of which push the cost up relative to a dormer of similar floor gain.
A rear dormer's cost is driven mainly by its size, whether it is flat-roofed or has a pitched cheek, and how much of the existing rear slope needs stripping back to tie the new structure in cleanly.
Common questions
Can I do a rear dormer without touching the hip?+
Yes, and on many houses that is enough if the landing already has adequate headroom. The hip only becomes a problem where the stair or a bathroom needs to sit close to the gable end.
Does hip-to-gable always need planning permission?+
Not always — it is often permitted development on its own. Combining it with a large rear dormer, or building on a corner plot with two visible elevations, is what commonly pushes a scheme into needing full permission.
Which is quicker to build?+
A rear dormer alone is usually the shorter programme because less of the existing roof structure is disturbed. Hip-to-gable adds masonry or framing and structural steelwork that extend the programme.
Get a loft conversion assessment
We'll survey the existing hip and roof structure and tell you plainly whether a dormer alone gets you the room you want, or whether hip-to-gable earns its keep.