Loft Conversion Cost London — Honest 2025 Guide
Real prices from real London projects. Not national averages. Not ballpark guesses. We've built hundreds of loft conversions across all London boroughs — here is what they actually cost.
Loft conversion costs by type
All figures are based on typical London projects including labour, materials, structural engineer coordination, and Building Regulations. Second-fix materials, carpet, and planning fees are separate.
Velux / Roof Light
£18,000 – £30,000The most cost-effective option. Adds light and headroom using roof windows built into the existing slope — the roof structure stays unchanged. Works best where head height already exists.
Typical use: Studio / office / storage conversion. No dormer, no planning permission in most cases.
- ✓Structural floor upgrade (typically 302mm joists)
- ✓Velux windows (typically 2–3)
- ✓Staircase (straight or space-saver)
- ✓Insulation — rafters and floor
- ✓Electrics — sockets, lighting, smoke detection
- ✓Boarding and plastering
- ✓Loft hatch and loft ladder (if existing access is retained)
- ✗En-suite / bathroom
- ✗Dormer
- ✗Any structural roof alterations
Dormer (Rear)
£35,000 – £55,000The most popular London loft conversion. A flat-roofed or pitched dormer extends from the rear slope, dramatically increasing usable floor space and allowing full-height headroom across most of the floor plate.
Typical use: 1-bedroom with en-suite. Permitted development (rear-only) in most London boroughs — no planning permission needed.
- ✓Structural floor upgrade
- ✓Rear dormer structure, cladding and flat roof
- ✓Dormer windows (typically 2–3 casements or fixed)
- ✓En-suite bathroom (first-fix, tiling, suite)
- ✓Staircase
- ✓Full insulation package
- ✓Electrics, lighting, heating
- ✓Plastering and boarding throughout
- ✓Building Regulations — structural engineer, drawings, applications
- ✗Second-fix materials (sanitaryware, fixtures, fittings) — client-supplied
- ✗Carpet — RCB does not supply or fit carpet
- ✗Fitted wardrobes or bespoke joinery
Hip-to-Gable
£40,000 – £60,000Typically combined with a rear dormer on semi-detached or detached properties. The sloping 'hip' of the roof is replaced with a vertical gable wall, significantly widening the usable floor area.
Typical use: Maximises floor area on semi-detached properties. May require planning permission if the property is end-of-terrace or in a conservation area.
- ✓Hip-to-gable structural alteration
- ✓New gable wall — brick or timber frame
- ✓Rear dormer (usually combined)
- ✓Full insulation, electrics, plastering
- ✓Building Regulations and structural engineer
- ✗Planning fees if required
- ✗Second-fix materials (client-supplied)
- ✗Carpet
L-Shaped Dormer
£55,000 – £80,000An L-shaped dormer combines a main rear dormer with a smaller side dormer over the rear return — common on Victorian and Edwardian terraces in London. Creates substantially more usable space than a simple rear dormer.
Typical use: 2 rooms + en-suite. Excellent use of the full roof footprint on terraced properties. Usually permitted development — no planning in most cases.
- ✓Main rear dormer + side return dormer
- ✓Connection roof structure between the two
- ✓2 rooms or rooms plus bathroom
- ✓Full structural, M&E, insulation, plastering package
- ✓Staircase
- ✗Second-fix materials (client-supplied)
- ✗Carpet
- ✗Bespoke joinery
Mansard
£70,000 – £120,000+The maximum loft extension — replaces the entire roof slope with near-vertical walls and a shallow-pitched or flat roof. Common in Central London and conservation areas, but almost always requires full planning permission.
Typical use: Maximum usable space. Near full-height walls throughout. Often used in conservation areas or high-value properties where planning allows.
- ✓Full mansard roof structure
- ✓Structural steelwork where required
- ✓Party wall works if required
- ✓Full M&E, insulation, plastering, joinery package
- ✗Planning fees and architect's design fees
- ✗Party wall surveyor fees
- ✗Second-fix materials
- ✗Carpet
What affects the cost of a loft conversion in London?
Eight factors that can move the final number — and by how much.
A Velux conversion and an L-shaped dormer are entirely different structural and building programmes. The type determines the base cost range.
Adding an en-suite adds £8,000–£14,000 to the scope. A second bathroom or larger space increases labour, M&E, and programme time.
A standard straight staircase costs significantly less than a bespoke winder or a space-saver with storage. Headroom and landing constraints drive this.
Most London loft floors need upgrading — standard ceiling joists aren't designed as a structural floor. The extent of this work varies with existing structure.
Larger dormers and standing-seam zinc, copper, or slate cladding cost more than a standard flat-roof EPDM dormer.
Most rear dormers in London are permitted development. Mansards and some hip-to-gable conversions need planning — adds cost and time.
If your work affects a shared wall or you are within 6m of a neighbour's foundation, a party wall agreement is required. Surveyor fees vary: typically £600–£2,000 per neighbour.
Properties with restricted access (no rear lane, no skip parking, dense urban location) add to the cost through manual handling and additional labour.
About these figures
These are ballpark ranges based on similar projects and current London market conditions (2025). They are not formal quotations. Actual costs depend on your specific property, structural condition, specification, and programme. A detailed estimate is only produced after a site survey and receipt of available drawings or technical information. Costs may be higher or lower once we have assessed your project properly — that's normal and expected. It is a refinement based on real data, not a contradiction of what you read here.
Loft conversion cost — frequently asked questions
How much does a loft conversion cost in London?
A London loft conversion costs £18,000–£120,000+ depending on type. Velux conversions start from £18,000. A standard rear dormer with en-suite runs £35,000–£55,000. L-shaped dormers typically cost £55,000–£80,000. Mansard loft conversions start at £70,000 and can exceed £120,000 on larger or more complex properties. These are real figures from London projects completed by RCB Design & Build — not theoretical national averages.
Does a loft conversion need planning permission in London?
Most loft conversions in London don't require planning permission — they are covered by Permitted Development rights. Rear dormers, Velux conversions, and hip-to-gable conversions on semi-detached or detached properties typically fall within PD limits. Mansard loft conversions almost always require full planning permission. Conservation areas, Article 4 directions, and certain listed buildings restrict PD rights — we check this at survey stage for every project.
Does a loft conversion need Building Regulations approval?
Yes — always. Building Regulations are required for all loft conversions regardless of planning status. This covers structural design (new floor, staircase, roof alteration), fire escape and smoke detection, thermal insulation, and electrics. RCB manages the Building Regulations application, structural engineer coordination, and all inspections. You receive a completion certificate at the end.
How long does a loft conversion take?
A Velux conversion typically takes 4–6 weeks on site. A rear dormer conversion takes 8–12 weeks. An L-shaped dormer or hip-to-gable typically takes 10–14 weeks. A mansard conversion can take 16–24 weeks depending on complexity and any structural requirements. Programme is discussed and agreed before we start — you receive a written schedule.
Do I need to move out during a loft conversion?
Usually not. Most loft conversions can be delivered with the property occupied. Noise, dust, and access disruption are managed, but the works are largely contained to the loft level and the new staircase zone. We discuss this at the pre-construction stage and agree the most practical approach for your household.
Does a loft conversion add value?
Yes. A well-executed loft conversion typically adds 15–25% to a London property's value, depending on location and quality. The value uplift is strongest on properties that are undervalued relative to comparables that already have loft rooms — which describes a large proportion of London Victorian and Edwardian terraces.
What is not included in an RCB loft conversion price?
Our loft conversion prices include all labour, materials for the structure and first-fix, a standard bathroom suite allowance as defined in the scope, and Building Regulations. They exclude second-fix materials you choose yourself (sanitaryware, tiles, light fittings, ironmongery), carpet (which we do not supply or install), bespoke joinery unless specified, and planning or party wall surveyor fees if applicable.
Get a free estimate for your loft conversion
Tell us about your property and what you're planning. We'll come back within a few hours with a realistic budget range and arrange a free site visit if you'd like to go further.