For owners of period London houses planning a new top floor
Loft Conversion Cost in London: What Drives It
Sviatoslav JerdetchiiCEO & Founder of Craftex. Updated , 4 min read
The short answer
A loft conversion cost in London is set by the type of conversion, the structure, the stair and fire route, and the finish. The Home Owners Alliance puts typical conversions at around £1,250 to £2,500 per m², and a mansard suite on a prime period house can sit at the top of that range or above it.
- A rooflight conversion costs least; a dormer more; a mansard, which rebuilds the roof, the most.
- Many London houses are in conservation areas, where loft permitted development does not apply and planning decides the design.
- Steel, the new stair and a protected escape route are the hidden half of the cost.
- A fixed price follows a survey, drawings and a written scope; add VAT, fees and a contingency.
A loft conversion cost in London is set by four things: the type of conversion, the structure, the stair and fire route, and the finish. The Home Owners Alliance puts typical conversions at around £1,250 to £2,500 per m², depending on the design, the builder and the location.
On a prime period house, a mansard suite with a bathroom and bespoke joinery can sit at the top of that range or above it. This guide explains why, type by type, and what else sits in the budget.
Planning a new top floor? Our loft conversion in London service designs, gets the consents for and builds mansard and loft suites as one team. Discuss your project and we will look at the roof with you.
Dormer, hip-to-gable and mansard loft conversions
The type of conversion is the biggest single choice in the cost. Each step down the table adds structure, roofing and floor area.
| Type | What changes | Typical fit |
|---|---|---|
| Rooflight | Roof windows set into the existing slope; the shape stays | Lofts with good head height already |
| Dormer | A box projects from the rear slope to add head height | Most terraces and semi-detached houses |
| Hip-to-gable | A sloping side roof is built up into a vertical gable | End-of-terrace, semi-detached and detached houses |
| Mansard | The rear slope is rebuilt as a near-vertical wall with a flat top | Period townhouses; the most floor area and head height |
HOA's averages put the mansard as the most expensive of the four, and it is also the one that adds the most usable space. On a Georgian or Victorian townhouse a mansard often reads as part of the original house, which is why conservation officers tend to prefer it.
Permitted development or planning permission
A loft can be permitted development, which saves the planning application but not the rules. The limits are strict, and in much of central London they do not apply at all.
- Volume. Up to 40 m³ of new roof space on a terrace, or 50 m³ on other houses.
- Height and front. Nothing above the highest part of the roof, and nothing beyond the front slope facing the street.
- No balconies. Verandas, balconies and raised platforms are excluded.
- Protected land. Not in a conservation area, and the right applies to houses, not flats.
Most London mansards therefore need planning permission. Our guide to permitted development extension rules covers the wider household rights, and planning and building regulations drawings explains the two sets of drawings.
Structure, stairs and the fire escape
This is the hidden half of a loft budget. None of it shows in the finished room, and all of it is checked by building control.
- Floor and steels. Ceiling joists are rarely strong enough to be a floor, so new joists and usually steel beams carry the new storey.
- Party walls. Raising or building onto a shared wall needs party wall notice at least two months ahead.
- The stair. A full stair, not a ladder, has to land where it leaves head height and does not take a bedroom.
- Fire escape. A new storey needs a protected route down through the house, usually fire doors and linked alarms.
- Insulation. The new roof and walls must meet the energy rules in the Building Regulations.
Building Regulations approval is needed for every loft conversion, whether or not planning is.
What drives the cost of a loft conversion
Once the type is chosen, these decide where in the range your loft lands.
- Floor area and head height. More usable area costs more in total, but often less per square metre.
- Bathroom. A new en suite means drainage, a soil stack and waterproofing at the top of the house.
- Specification. Joinery, glazing, stone and heating: the same choices that set the rest of the house.
- Access. Scaffolding, a pavement licence and deliveries up a London street.
- Planning conditions. Slate, lead and timber sash windows specified by the conservation officer.
- The house below. Making good ceilings, the stair well and the services on the floors beneath.
Our build cost calculator gives a first indication. Add VAT at 20%, design and engineering fees, and a contingency of 5% to 10% of the construction cost.
A loft inside a whole-house renovation
On a prime house the loft is rarely the only work. Many owners add a mansard suite while they renovate the rest of the house, and that changes the sums.
One scaffold, one set of consents and one programme cover both. The stair, the heating and the electrics are designed once for the whole house. Our guide to house renovation cost in London explains how the rest of the budget is built.
At Kensington Olympia we redesigned and rebuilt a 400 sq m top-floor penthouse in 12 weeks, inside an occupied building, with no snags at handover. The same discipline runs a mansard: the design is fixed and long-lead items are ordered before the roof comes off.
This was a full high-end renovation of my Victorian rental apartment in Westminster, just off Sloane Square and Belgravia. The scope included full fit-out: electrics, plumbing, a new bespoke kitchen, and a luxury bathroom refurbishment. Craftex took care of everything and made the process easy. They weren’t the cheapest, but you can tell they really know what they’re doing when it comes to prime London property renovations. The team was professional, organised, and genuinely seemed to care about getting everything right. I was especially impressed with the quality of the finish – the attention to detail really shows in every corner. I’m very happy with the results and would definitely recommend Craftex to anyone looking for reliable, high-quality renovation work in Central London.
Loft conversion timescales
The build is the shorter part of a loft project. Most of the time goes into what comes before it.
- Survey and design. Measured survey, structural check and the design options.
- Planning or a lawful development certificate. The council's decision, or a certificate confirming permitted development.
- Party wall and building regulations. Notices to neighbours and the technical drawings.
- Build. Scaffold, roof opened, structure, weathertight, then stair, services and finishes.
- Sign-off. Building control completion and handover.
A mansard takes longer on site than a dormer, because the roof is rebuilt rather than extended. Have a house and a brief? Discuss your project and we will reply the same working day.
