A guide to loft conversions in London

A loft conversion is the most efficient extra room in London. The footprint already exists, the roof is already there, and in the right house you gain a bedroom and bathroom without losing a square metre of garden. It is also, structurally and legally, more involved than most homeowners expect.

This guide covers the four common types, how to judge whether your roof is viable, the structural and fire requirements, and what really drives the cost.

The four types

Rooflight conversion

The roof structure stays as it is; you insulate, floor, install rooflights and build a staircase. The simplest and least disruptive option, and usually the only one available where planning constraints are tight. It works only where the existing head height is already generous.

Dormer

A flat-roofed box built out from the rear roof slope, creating full-height usable space and room for a proper staircase and window. The standard London solution on terraces and semis, most often to the rear where permitted development or local policy allows.

Hip-to-gable

On a semi-detached or end-terrace house with a hipped side roof, the hip is rebuilt as a vertical gable wall, squaring off the plan. Almost always combined with a rear dormer to produce a genuinely usable floor.

Mansard

The roof is largely rebuilt with a steep, near-vertical rear slope and a small flat top, producing the most floor area of any option — often a full extra storey. It is the most structurally significant and the most expensive, and in conservation areas it is heavily governed by the borough's design guidance. Assume a planning application and check current local policy carefully.

Is your roof viable?

Start with a tape measure and the ridge. Measure from the top of the existing ceiling joists to the underside of the ridge board at the highest point.

• Below about 2.2m, a conversion without altering the roof is unlikely to work; you are looking at a dormer, mansard or roof raise.
• Around 2.2m to 2.4m, a conversion is usually feasible, but the new floor structure and insulation will consume some of it.
• Above 2.4m, you have comfortable options.

Also check the roof structure itself. A traditional cut roof with purlins and rafters is straightforward to open up. A trussed rafter roof, common in later twentieth-century houses, requires the trusses to be removed and replaced with a new structural floor and roof support — possible, but a bigger engineering exercise.

Then look at the obstacles: the water tank position, the chimney breasts, where the staircase can land without ruining the floor below, and whether the head height over the new stair can be achieved.

Structure

A loft conversion turns a storage void into a habitable floor, and the building has to be told about it. Typically that means:

• New floor joists or steel beams spanning between load-bearing walls, independent of the old ceiling joists.
• Steelwork at the ridge and eaves where the roof is opened up for a dormer or mansard.
• Padstones and, occasionally, strengthening of the walls or foundations below where new loads are concentrated.
• A structural engineer's design and calculations, submitted to building control.

Where you share a party wall — which in a London terrace you always do — cutting in beam bearings triggers the Party Wall etc. Act 1996 and its notice periods. This is covered under our structural modifications service.

Stairs and fire safety

This is where loft conversions are most often designed badly, and it is not negotiable.

Adding a habitable storey usually makes the house three storeys, which changes the fire strategy for the whole building under Approved Document B. In practice that commonly means:

• A protected escape route: the stairway enclosed in fire-resisting construction from the top storey down to a final exit.
• Fire doors with self-closing devices to habitable rooms opening onto that stairway.
• Mains-powered, interlinked smoke alarms on every storey.
• Escape windows to lower-storey rooms where required, or an alternative approach agreed with building control.
• A compliant staircase under Approved Document K — a proper rise and going, adequate headroom over the pitch line, and handrails. Space-saving alternating-tread stairs are permitted only in narrowly defined circumstances and should never be assumed.

Where the geometry genuinely will not allow a protected stair, a sprinkler or mist suppression system is sometimes agreed as part of an alternative fire strategy. That is a conversation to have with building control at design stage, not on site.

What drives the cost

Broad indicative ranges for London, 2026, for a completed conversion including structure, stair, insulation, glazing, plastering, electrics, heating, decoration and a simple en-suite:

• Rooflight conversion: approximately £45,000 to £75,000.
• Rear dormer: approximately £65,000 to £110,000.
• Hip-to-gable with rear dormer: approximately £85,000 to £140,000.
• Mansard: approximately £120,000 to £200,000 and upward.

Treat these as orientation only. The variables that move the number most are:

1. Structural complexity — trussed roofs, long spans, chimney removal, foundation strengthening.
2. Glazing — the difference between standard rooflights and large bespoke units is substantial.
3. Bathroom specification — sanitaryware, stone, underfloor heating and the pump or booster needed to serve a top floor.
4. Access — scaffolding, crane hire, restricted street frontage, permits and bay suspensions.
5. Finishes — fitted joinery, panelled walls and made-to-order doors, all of which carry lead times as well as cost.
6. Professional fees — architect, structural engineer, party wall surveyor per adjoining owner, building control.

Add VAT at the prevailing rate, professional fees and a contingency of at least ten per cent on top of any construction figure.

How we manage a loft conversion

The awkward part of a loft is that it happens above your head while you live underneath it. Sequencing matters: scaffold and weatherproofing first, structure and roof opened and closed in a controlled window, then a clean, dust-managed route through the house for everything that follows.

We hold the design team, the engineer, the party wall process and the trades on one programme through project management and coordination, take the consents route through planning permissions and approvals, and finish with fitted joinery and detailing under custom finishes and bespoke solutions.

See how loft floors read in a finished house across our projects, or tell us about yours.

Indicative guidance for 2026, not professional advice. Costs, regulations and local planning policy change; confirm the position for your property with your local authority, building control body and appointed consultants.