A loft conversion is one of the most cost-effective ways to add living space to a home. Unlike an extension, you are using existing roof space rather than your garden, and there is no requirement for planning permission in most cases. Here is a comprehensive breakdown of costs by conversion type for 2025.
Velux / Rooflight
£15,000 – £25,000
No structural roof changes. Rooflights inserted into existing slope. Lowest cost, least space.
Rear Dormer
£35,000 – £55,000
Box-shaped extension from rear roof slope. Most popular type. Good headroom gain.
Hip-to-Gable
£40,000 – £65,000
Converts hipped end into vertical gable wall. Common on detached and semi-detached houses.
Mansard
£50,000 – £80,000
Rebuilds rear roof at 72° angle. Maximum space. Usually needs planning permission.
L-Shaped Dormer
£55,000 – £90,000
Two dormers forming L-shape. Ideal for Victorian/Edwardian terraces. Most space of any dormer type.
Location Adjustments
As with all building work, costs vary significantly by region. London and South East conversions typically cost 30-60% more than equivalent work in the Midlands or North of England. The cost ranges above are for the Rest of England; apply these multipliers:
London: +40-60% above Rest of England
South East: +20-35% above Rest of England
Scotland / Wales: -10-20% below Rest of England
Additional Costs for a Loft Conversion
Beyond the main build cost, budget for these additional items:
Architect / technician drawings: £1,500 – £3,000 for planning and building regs drawings
Structural engineer: £500 – £1,200 for steel beam calculations and drawings
Building regulations fee: £700 – £1,500 (local authority or approved inspector)
Party wall notice: Free if neighbours agree; £700 – £2,000 per surveyor if dissent
Staircase: £2,000 – £6,000 depending on design (space-saver or full staircase)
Ensuite bathroom: £4,000 – £10,000 for supply and installation
VAT: 20% on all contractor costs
Contingency: 10-15% above the quoted price
Staircase: A Key Design Challenge
The staircase to the loft conversion is often the most design-challenging element. Building regulations require a minimum headroom of 1.9m above the staircase line — many older properties barely meet this. A space-saving alternating-tread stair can be used in some circumstances, but a conventional staircase is far preferable. The staircase location also dictates how much space is lost from the floor below — a critical consideration in smaller homes.
Feasibility Check: Before commissioning full plans, measure the height from the top of your ceiling joist to the underside of the ridge beam. If this is 2.2m or more, the conversion is almost certainly feasible. Between 1.9m and 2.2m it may still work with careful design. Below 1.9m it is unlikely to be possible without significant structural work.
Planning Permission for Loft Conversions
The majority of loft conversions in England are Permitted Development — meaning they do not require a full planning application. The key rules are:
Volume added must not exceed 40m³ for terraced houses, or 50m³ for detached and semi-detached houses
No extension beyond the plane of the existing roof slope on the principal elevation (front)
Rear dormers must not exceed the highest part of the existing roof
Materials must be similar in appearance to the existing house
Planning permission IS required for:
Mansard conversions (which effectively create a new roof structure)
Properties in Conservation Areas, National Parks, or Areas of Outstanding Natural Beauty
Any loft conversion where PD rights have been removed by an Article 4 Direction
Flats and maisonettes (no PD rights)
Fire Safety Requirements
Building regulations have specific fire safety requirements for loft conversions, particularly in three-storey properties:
Mains-wired interlinked smoke alarms on every floor (including the loft level)
Fire doors (FD30) on all habitable rooms on the floors below the loft
A protected escape route from the loft room to ground level
In some cases: an escape window (at least 450mm x 450mm with 0.33m² clear opening) in the loft room
Important: Fire safety requirements are non-negotiable and will be inspected by building control. Never try to save money by skipping fire doors or smoke alarms — these are life-safety requirements.
How Much Value Does a Loft Conversion Add?
Adding a bedroom through a loft conversion typically increases property value by 10-15%. An ensuite master bedroom suite can push this to 20% in competitive property markets. The key drivers of value uplift are:
Moving from a 2-bed to a 3-bed property (significant price bracket jump in most areas)
Moving from a 3-bed to a 4-bed property
Adding an ensuite bathroom to the master bedroom
Quality of finish matching or exceeding local expectations
Natural light — a poorly-lit loft room adds less value than a bright, well-fenestrated space
On a £400,000 property, a 12.5% uplift represents £50,000 of added value. A typical dormer conversion costing £55,000 all-in could therefore offer strong financial returns in the medium term — especially compared to moving home (with its substantial transaction costs).
Frequently Asked Questions
How much does a loft conversion cost in the UK in 2026?
Loft conversion costs range from £15,000-£25,000 for a basic Velux conversion to £50,000-£90,000 for a full mansard or L-shaped dormer. Dormer conversions — the most popular type — typically cost £35,000-£55,000 including VAT and professional fees. London costs are 40-60% higher than the national average. Always add a 10-15% contingency budget.
Do loft conversions require planning permission?
Most loft conversions in England fall under Permitted Development and do not need planning permission, provided the volume added does not exceed 40m³ for terraced houses or 50m³ for detached and semi-detached. Mansard conversions and those in conservation areas typically require full planning permission. A Certificate of Lawful Development (£103) provides legal certainty before you begin.
How much value does a loft conversion add?
Adding a bedroom through a loft conversion typically increases property value by 10-15%. An ensuite bedroom can add up to 20% in desirable areas. The uplift depends heavily on your local property market and whether adding a bedroom moves you into a significantly higher price bracket. Moving from 2-bed to 3-bed typically adds the most proportional value.
What is the minimum headroom needed for a loft conversion?
Building regulations require a minimum headroom of 1.9m above the staircase for a loft conversion to be habitable. The ideal is 2.2m+ at the apex. You can measure from the top of the ceiling joist to the underside of the ridge to get an initial indication of feasibility. Below 1.9m, a conversion may still be possible with structural changes to raise the ridge or lower the ceiling below.
What is the difference between a dormer and a mansard loft conversion?
A dormer conversion adds a box-shaped protrusion from the existing roof slope to create headroom, typically on the rear. A mansard conversion involves rebuilding most of the roof to create near-vertical rear walls (72° angle) and a flat roof — maximising usable space but at higher cost and usually requiring planning permission. Mansard conversions are most common in London and on Victorian terraced properties.
Do I need fire safety measures for a loft conversion?
Yes. Building regulations require mains-wired interlinked smoke alarms on every floor, fire doors (FD30) on the loft landing and all habitable rooms on the floors below, and a protected escape route from the loft. For a 3-storey property, an escape window in the loft room may also be required. These are enforced by building control at inspection stages.
How long does a loft conversion take?
A Velux loft conversion typically takes 4-6 weeks on site. A dormer conversion takes 6-10 weeks. A full mansard conversion can take 10-16 weeks. Add design, planning (if required) and building control approval time — typically 2-4 months before work can start. Most reputable loft conversion companies are booked 2-4 months in advance.