Loft Conversion Cost in London (2026 Guide)

How much does a loft conversion cost in London in 2026?

Build costs depend mostly on the conversion type. A rooflight (Velux) loft is the cheapest because it changes nothing structurally outside the roof and simply fits out the existing space; a rear dormer sits in the middle, adding a box of usable floor area at the back; and a mansard — often the only form the planners will accept in a conservation area — is the most expensive because it effectively rebuilds the whole rear roof slope at a steep 70-degree pitch.

London costs run above the national average for the same reasons everything does here: access is tight, skips and scaffolding cost more, party walls are almost universal on terraces, and labour is at a premium. The ranges in the table below are realistic 2026 figures for the shell and a standard finish; a high-end en-suite, bespoke joinery or structural glazing will push a project above them. It is worth pricing the conversion type that suits your roof rather than reaching for the largest — a dormer that gives you the bedroom you need is far better value than a mansard you built because it sounded grander.

What do loft conversion drawings and approvals cost?

Separately from the build, you need drawings and approvals: a permitted development or planning route, building regulations drawings, and structural calculations. As a fixed fee these typically run £1,100–£2,500 depending on whether the loft is permitted development or needs a full planning application, and on how much structural design the engineer has to do for the new floor, the dormer and any beams supporting the roof.

That fee buys three distinct things. First, the planning stage — either a Lawful Development Certificate proving the loft is permitted development, or a full householder application if it is not. Second, the building-regulations drawings that show how the conversion is actually built to be safe and warm: the escape stair, the fire doors, the insulation and the structure. Third, the structural calculations for the steel beams and new floor joists. A cheap quote that covers only the planning drawings is not comparable to a complete package, because you will still have to buy the building-regs and structural work before a builder can start.

How can I keep a London loft conversion on budget?

The biggest savings come before the build, not during it. Three decisions do most of the work: confirming permitted development eligibility so you may avoid the planning route and its delay; choosing the conversion type that suits your roof rather than over-building; and getting accurate, complete structural drawings so every builder prices the same specification and there are no mid-project variations. The single most common cause of overspend is a thin set of drawings that a builder has to interpret — each ambiguity becomes either a padded contingency in the quote or a change order once work starts. Paying properly for the drawings is how you protect the far larger build budget behind them.

Which loft conversion type gives the best value in London?

The best-value conversion is the one that suits your roof, not the largest one you can build. A rear dormer usually offers the most usable space for the money on a typical terraced or semi-detached roof, while a rooflight loft is cheapest but adds the least headroom. A mansard costs the most because it rebuilds the roof, so it earns its place mainly where a conservation area rules the cheaper options out. Matching the type to the roof means you are not paying for structure you do not need.

The hip-to-gable is the useful middle option for semi-detached and end-of-terrace houses: it squares off a sloping hip end into a vertical gable wall, unlocking floor area that a plain dormer cannot reach, usually combined with a rear dormer for the bedroom itself. On a mid-terrace with two party walls a hip-to-gable is not available, so the rear dormer does the heavy lifting. The honest way to choose is to start from what the roof geometry allows and what the planners will accept on your street, then pick the cheapest type that delivers the room you actually need — not to start from a type and force the roof to suit it.

What hidden costs should I budget for in a loft conversion?

Beyond the build and the drawings, budget for the extras that catch people out: the council's own planning application fee, which is separate from drawing fees; the Building Control charge, paid to the council or an approved inspector for inspecting the work; a party-wall surveyor where a neighbour does not consent in writing; and the finishing trades — flooring, decoration, heating and the staircase — that turn a shell into a room.

Two more often slip the budget. The staircase itself is a real cost and a real design problem, because a compliant loft stair has to land somewhere on the floor below without eating a bedroom. And if the roof needs raising or the party wall building up, the structural scope grows. Accurate structural drawings protect the budget against all of this, because they let the builder price precisely against a fixed specification instead of loading a contingency for the unknown — which is exactly where a vague drawing package costs you far more than it saved.

What decides how much of the roof you can actually use?

Two lofts of the same footprint can deliver very different rooms, and the difference is usually headroom and how the regulations shape the space rather than the raw square metres. Before you set a budget it pays to understand what the roof will physically allow, because it changes both the conversion type you should choose and the price you should expect. A quick loft assessment against these two factors is the single most useful thing you can do before commissioning drawings.