Loft Conversions in London: Dormer, Hip to Gable or Mansard?

Three loft types dominate London roofs, and the one your house can take is usually decided by the roof itself before budget or preference get a say. Most loft conversions in the capital come down to a choice between a dormer, a hip to gable and a mansard, and knowing which suits your roof shape saves a wasted design fee.

Here is how the three compare, and why the dormer wins on so many London streets.

The Roof Decides Before You Do

Start with two measurements, because they narrow the field faster than anything else.

Head height first. Measure from the top of the ceiling joists to the underside of the ridge. Around 2.4 metres is what the trade works to, since the new floor and ceiling both eat into it. There is no statutory minimum ceiling height for a habitable loft room in England, so figures quoted online are convention rather than law. What is actually regulated is headroom over the staircase.

Roof shape second. A hipped roof slopes on three or four sides. A gable roof ends in a vertical triangular wall. That single distinction decides whether a hip to gable is even available to you.

Once you know both, the choice is usually down to one or two options rather than three.

The Dormer: London’s Default

A dormer cuts a section out of the rear roof slope and builds a box out from it, with vertical walls and a flat roof. That vertical wall is the whole point, because it converts sloping dead space into full height room.

It is the most common conversion in London for good reason. It suits terraced, semi detached and detached houses alike, and a rear dormer usually falls under permitted development, which removes the planning application from the process entirely.

Cost sits around £45,000 to £75,000 for a standard rear dormer, with inner London and higher specifications pushing toward the top of that range. Per square metre that is strong value, because you are modifying the roof rather than rebuilding it.

Under Class B of the GPDO, a terraced house is allowed 40 cubic metres of additional roof space and a semi detached or detached house 50 cubic metres, where permitted development rights survive. Volume is cumulative, so any previous roof addition counts against the allowance. A dormer loft conversion that stays within these limits, sits below the existing ridge and is set back from the eaves usually avoids a planning application altogether.

The one variation worth knowing is the L shaped dormer, which extends over the main roof and the rear addition of a Victorian or Edwardian terrace. It produces the most space of any dormer arrangement, because it uses the back addition that terraces already have.

The Hip to Gable: Only With a Hipped Roof

A hip to gable removes the sloping hip end of a roof and rebuilds it as a vertical gable wall in brick. That squares off a corner that was previously sloping dead space, and it opens up room that a dormer alone cannot reach.

It only exists as an option if your roof has a hip to convert. On a mid terrace with a gable already, it is off the table. On an end of terrace, semi detached or detached house with a hipped roof, it is often the strongest single move available, especially combined with a rear dormer.

Cost runs higher than a dormer, roughly £55,000 to £95,000 in London, because you are rebuilding a full roof slope in masonry rather than adding a box to it. That extra structural work is the reason for the gap.

Most hip to gable conversions on semi detached houses fall under permitted development, though detached houses may need permission depending on the scale, so the address level check matters.

The Mansard: Most Space, Most Process

A mansard rebuilds the rear roof plane to a near vertical slope of around 72 degrees, with a flat roof on top. It effectively adds a full extra storey, and it delivers more usable floor area than any other type.

That space comes at the highest cost and the most process. London figures run roughly £60,000 to £120,000, and inner London conservation area mansards in natural slate with lead detailing run higher still, because conservation specification alone can add 15 to 25 per cent over standard materials.

Mansards almost always need full planning permission. They are common on inner London terraces, particularly in conservation areas, where they are often the only way to add significant space and where they can suit a street that already has them. On a terrace of intact original roofs, a mansard that breaks the line is a harder case.

Comparing the Three

Type	Typical London cost	Requires	Planning
Dormer	£45,000 to £75,000	Adequate ridge height	Often permitted development, rear only
Hip to Gable	£55,000 to £95,000	A hipped roof	Often permitted development on semis
Mansard	£60,000 to £120,000	Suitable terrace context	Almost always full permission

What Every Type Needs Regardless

Two Building Regulations shape all three, and both are worth pricing in from the start.

Approved Document K governs the staircase. The standard requirement is 2 metres of headroom over the stair, but loft conversions get a specific relaxation to 1.9 metres at the centre reducing to 1.8 metres at the side, where a conforming stair is otherwise impossible. Even so, a compliant staircase takes more floor area than people expect, and that area comes out of a first floor bedroom.

Approved Document B covers escape. Adding a second floor to a house, taking a new floor above 4.5 metres from ground level, triggers the need for a protected stair enclosure with fire doors to the rooms opening onto it. That is cost in rooms you were not otherwise touching, and it applies to every loft type.

Which One Actually Adds the Most Value

All three add value, and the mechanism is the same. A loft that turns a three bedroom house into a four bedroom one, with a genuine principal bedroom and en suite, moves the property up a bracket that buyers search by.

Nationwide research has put the uplift from adding a bedroom and bathroom in the loft at around 20 per cent of property value, which on London stock comfortably clears the build cost of any of the three types. The effect is strongest where the new room is a real double with proper headroom, and weakest where compromises leave it a study with a bed in it.

That is the argument for choosing the type that genuinely fits the roof rather than the cheapest available. Extension Architecture’s approach is to establish the roof geometry and the consent route before any layout work begins, because a conversion designed against the wrong type is a conversion drawn twice.