Loft Conversion Cost Calculator
Enter your property type, conversion type, floor area and whether you need an en-suite to instantly estimate your loft conversion cost, from a simple Velux rooflight conversion to a full mansard or hip-to-gable build.
Property Specs
Roughly £2,332 per square metre
A Guide to Loft Conversions
Moving house in the UK has become incredibly expensive. Between stamp duty, estate agent fees, and legal costs, you can easily spend £20,000+ just on the act of moving. That is why extending your current home with a loft conversion is often the smartest financial decision you can make. It unlocks dead space, gives you an exceptional master suite or home office, and significantly increases your property value.
Understanding the Hidden Costs
When builders quote for a loft conversion, their initial figure often excludes vital elements that you are legally required to pay for. Our calculator includes these to give you a realistic final bill.
- Professional Fees: You must hire a structural engineer to ensure the new floor joists and roof alterations are safe. You also need architectural drawings, even for permitted development. (Usually 10% of build cost).
- Building Control: The local council or a private inspector must sign off the work to prove it complies with fire safety and insulation regulations. This costs around £700-£1000.
- Party Wall Agreements: If you share a wall with a neighbour (semi-detached or terraced), you must serve them notice. If they dissent, you pay for their surveyors too (£1,000 - £2,000+).
The 4 Main Types of Loft Conversion
1. Roof Light / Velux Conversion
The most cost-effective solution. You don't alter the shape of the roof; instead, you add skylight windows to the existing slant. This relies entirely on you already having adequate head height in the centre of your loft (at least 2.2m from floor joist to ridge board). Best for adding a simple home office or small spare room.
2. Dormer Conversion
The undisputed most popular choice in the UK. A dormant extends horizontally out of the existing roof slant, creating a box-like structure. This instantly gives you vertical walls and a flat ceiling, massively increasing the usable floor space and head height. You can usually fit a large master bedroom and an en-suite inside a dormer.
3. Hip-to-Gable Conversion
Common on semi-detached houses with a hipped roof (a roof that slopes down on the side as well as the front and back). This conversion involves extending the sloping side wall straight up to form a vertical 'gable' end. This is a massive structural alteration but completely opens up the stairwell and loft space.
4. Mansard Conversion
Often seen in central London and on terraced houses. A mansard essentially replaces one entire side of your roof (usually the rear) with an almost vertical wall (sloping at 72 degrees) and a flat roof across the top. It provides the maximum amount of space possible, practically building a whole new storey, but almost always requires planning permission and is the most expensive option.
Loft Conversion FAQs
Loft Conversion Cost Calculator UK
How Much Does a Loft Conversion Cost in the UK?
Loft conversion costs in the UK range from £22,000 for a basic Velux rooflight conversion to £100,000 or more for a full mansard, based on Q2 2026 live pricing data. A standard rear dormer, the most popular type, costs £40,000 to £65,000 for most UK properties outside London.
| Conversion Type | UK Cost Range (2026) | London Cost Range |
|---|---|---|
| Velux (rooflight) | £22,000 – £40,000 | £30,000 – £50,000 |
| Rear dormer | £40,000 – £65,000 | £50,000 – £90,000 |
| L-shaped dormer | £55,000 – £90,000 | £65,000 – £110,000 |
| Hip-to-gable with dormer | £50,000 – £80,000 | £65,000 – £100,000 |
| Mansard | £55,000 – £95,000 | £70,000 – £120,000+ |
| Bungalow roof lift | £45,000 – £95,000 | N/A |
| All figures include builder costs, structural steelwork, insulation, staircase, plastering and basic decoration. They exclude en-suite bathroom (add £8,000 to £15,000), bespoke glazing, architect fees, structural engineer calculations and Building Control fees. |
London and the South East add 20 to 40 percent to national average costs throughout.
Loft Conversion Types
Velux (Rooflight) Conversion is the cheapest and quickest type. It keeps the existing roof structure entirely intact and installs roof windows into the existing slopes. The loft is insulated, floored, plasterboarded and connected via a new staircase. It works only where existing head height is at least 2.2 metres at the ridge. Build time is 4 to 6 weeks and it almost always qualifies as permitted development.
Rear Dormer is the most common loft conversion in the UK. A box-shaped structure projects vertically from the rear slope, creating full headroom and full-height windows across the majority of the new floor area. It adds 25 to 40m² of usable space and takes 6 to 10 weeks. Most rear dormers qualify as permitted development under 40m³ for terraces or 50m³ for semi-detached and detached properties.
L-Shaped Dormer is the natural progression for Victorian and Edwardian terraced houses with a rear outrigger, the back addition common on late 19th century properties. Two dormer structures are built, one over the main roof and one over the back addition, creating an L-shaped floor plan. It delivers the most usable floor area of any dormer type and is particularly effective for creating a large bedroom with full en-suite.
Hip-to-Gable Conversion replaces the sloping triangular hip end of the roof with a vertical gable wall, then typically adds a rear dormer at the same time to maximise the resulting floor area. Available only on semi-detached, end-of-terrace or detached homes with a hipped roof, common in the 1930s semi-detached stock across the UK's suburban streets. It involves more structural work than a standard dormer and costs £10,000 to £15,000 more, but delivers 30 to 50 percent more usable floor area.
Mansard Conversion rebuilds the entire rear roof slope to a near-vertical 70 to 72-degree angle with a flat top, maximising head height and floor area throughout the new space. It creates the largest possible loft room and is the standard choice for London period terraces where maximum space utilisation matters. A mansard almost always requires full planning permission and is the most structurally complex and expensive type available.
Loft Conversion Cost Per m²
Build-only cost per square metre across conversion types in 2026:
| Conversion Type | Build Cost Per m² | Velux |
|---|---|---|
| £1,000 – £1,500 | Rear dormer | £1,200 – £1,900 |
| Hip-to-gable | £1,300 – £2,100 | Mansard |
| £1,700 – £2,500 | L-shaped dormer | £1,800 – £2,700 |
| Total project cost per m² including stairs, bathroom and decoration typically adds 15 to 20 percent on top of these build-only figures. |
Is Your Loft Suitable for Conversion?
Before committing to any type, three things determine whether your loft is convertible and which type is appropriate.
Head height is the most important check. Measure from the top of your ceiling joists to the underside of the ridge beam. A minimum of 2.2 to 2.4 metres is needed for a Velux conversion. If you have less than this, a dormer or mansard is required to create usable space, at additional cost.
Roof structure matters because modern trussed roofs, found in most homes built from the 1960s onwards, are more complex and expensive to convert than traditional cut rafter roofs, which can typically be adapted more straightforwardly. A trussed roof requires the trusses to be replaced or restructured by a structural engineer, adding £3,000 to £7,000 to the project cost.
Water tank position: older properties often have a cold water tank in the loft. Relocating or replacing it with a pressurised system is required before conversion and costs approximately £800 to £1,500.
Additional Costs to Budget For
Structural engineer calculations are not optional. Every loft conversion in the UK requires a structural engineer's calculation pack for Building Control sign-off, covering floor beams, ridge beams, steel specifications and gable or dormer wall structural elements. Fees start from £395 for simple conversions and typically run £800 to £1,500 nationally or £1,500 to £4,000 in London for complex projects.
Architect or designer fees add 5 to 10 percent of the build cost. An architect is not a legal requirement but is strongly recommended for mansard conversions, conservation area properties or any project requiring full planning permission.
Building Control fees are charged by the local authority or approved inspector and typically cost £500 to £1,000 for a residential loft conversion.
Staircase is mandatory for Building Regulations compliance. A standard staircase costs £1,500 to £3,000. Space-saving alternating-tread or ship's ladder stairs are permitted under building regulations only as secondary access to single rooms, not as the primary staircase.
En-suite bathroom adds £8,000 to £15,000 covering plumbing, sanitary ware, tiling and waste connection. This is almost always worth including at the time of conversion as retrospective plumbing through a finished loft is significantly more disruptive and expensive.
Party wall agreement: required if your conversion involves work on or within 3 metres of a shared wall with a neighbour. Budget £1,000 to £2,500 per affected neighbour for a surveyor-agreed party wall award.
Contingency: budget 10 to 15 percent. Hidden water tanks, undersized joists and unexpected structural complexity are the most common sources of overrun, particularly in older properties.
Planning Permission and Permitted Development
Most Velux and standard rear dormer conversions qualify as permitted development and do not require a planning application, provided they fall within volume limits, 40m³ for terraced houses and 50m³ for semi-detached or detached and do not extend above the existing ridge height or face a highway.
Mansard conversions and many hip-to-gable projects require full planning permission because they substantially alter the external appearance of the roof. Conservation area properties require planning permission for almost all visible roof alterations regardless of type.
A Lawful Development Certificate is strongly recommended even where planning permission is not required. It provides documented evidence that the conversion was lawful and is commonly requested by buyers' solicitors during future property sales.
How Much Value Does a Loft Conversion Add?
A loft conversion typically adds 10 to 24 percent to a UK property's value. On a £350,000 home that is a value increase of £35,000 to £84,000, often exceeding the conversion cost, particularly for dormer and hip-to-gable projects that add a full bedroom and bathroom.
Loft conversions consistently outperform single-storey extensions on return-on-investment because they add a bedroom to the property, the metric buyers pay most attention to, without reducing garden space. An extra bedroom moves a property from one price tier to another in most local markets, creating a step-change in value rather than a proportional uplift.
Loft Conversion vs House Extension
On a per-square-metre basis, loft conversions and single-storey extensions are broadly comparable at current 2026 prices, £1,200 to £2,500 per m² for a loft versus £1,900 to £3,300 per m² for a ground-floor extension.
Choose a loft conversion if you need an additional bedroom or bathroom, want to preserve your garden space and have an existing roof structure that is suitable. Loft conversions are also typically faster to plan and build than extensions.
Choose a house extension if you need more ground-floor space, a larger kitchen-diner or living room, since a loft conversion does not address ground-floor spatial problems. Extensions also tend to be simpler in planning terms for single-storey rear additions under permitted development.
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References
- BestBuilders. Loft Conversion Cost UK 2026. bestbuilders.co.uk, June 2026
- HouseUp. Loft Conversion Cost UK 2026. houseup.co.uk, June 2026
- Taskino. Loft Conversion Cost UK 2026. taskino.co.uk, June 2026
- Structural Engineer Calcs. Loft Conversion Cost UK 2026. structuralengineercalcs.com, May 2026
- MyJobQuote. Loft Conversion Cost Guide UK 2026. myjobquote.co.uk, June 2026
- Checkatrade. Loft Conversion Cost 2026. checkatrade.com
- Planning Portal. Permitted Development Rights for Loft Conversions. planningportal.co.uk
- SoftwareApplication