Cost calculator
Loft conversion cost calculator
Most UK loft conversions cost between £23,000 and £80,000 to build, before VAT and fees, depending on the type. In Berkshire and the rest of the South East, expect roughly £26,000 to £104,000. A typical 30m² rear dormer comes to £46,000 to £78,000 here, against £40,000 to £60,000 elsewhere.
Estimates based on published cost guides, last updated 22 September 2026. They help you budget; they are not quotes.
Estimated total project cost
£76,000 to £130,000
Build cost £57,000 to £91,000 before VAT. South East (including Berkshire), 30m².
Hip to gable, 1 bedroom with en-suite, standard finish.
Where the money goes
- Project management£4,600 to £7,300
- Scaffolding£5,500 to £8,600
- Internal preparation£8,100 to £12,500
- Roof structure alterations£5,800 to £9,100
- Steelwork£6,900 to £11,000
- Roof coverings£9,500 to £15,000
- Electrics and plumbing£3,700 to £5,900
- Joinery£5,200 to £8,200
- Staircase£3,200 to £5,000
- Plastering£3,200 to £5,000
- Decoration£2,000 to £3,200
- Build cost£57,000 to £91,000
- En-suite bathroom£4,600 to £13,000
- Lawful development certificate£274
- Party wall surveyor£900 to £2,700
- Building control£600 to £1,500
- Typical build time
- 10 to 14 weeks on site
- Planning
- This may be allowed under permitted development or may need planning permission, depending on the size and your property. Planning guide
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An estimate to help you budget, based on published cost guides. It is not a quote. Figures are reviewed and updated over time. How we calculate this.
Typical costs by type
| Conversion type | Floor area | Typical build cost | Build time |
|---|---|---|---|
| Velux or rooflight | 30m² | £26,000 to £39,000 | 4 to 6 weeks |
| Rear dormer | 30m² | £46,000 to £78,000 | 10 to 12 weeks |
| L-shaped dormer* | 30m² | £46,000 to £78,000 | 10 to 12 weeks |
| Hip to gable | 30m² | £57,000 to £91,000 | 10 to 14 weeks |
| Hip to gable with rear dormer* | 30m² | £57,000 to £91,000 | 10 to 14 weeks |
| Mansard | 30m² | £57,000 to £104,000 | 12 to 16 weeks |
| Bungalow | 50m² | £72,000 to £114,000 | 10 to 12 weeks |
Ranges, not quotes. Build cost only: excludes VAT, professional fees, planning and building control. Last updated September 2026.
* Estimated from the per-m² rate of a related type: no established source prices it separately.
Sources: Checkatrade, Loft conversion cost: 2026 UK price guide (March 2026 (table), August 2026 (breakdown)); Homebuilding & Renovating, Loft conversion costs (Tim Phillips, quantity surveyor) (29 July 2026).
| Conversion type | Floor area | Typical build cost | Build time |
|---|---|---|---|
| Velux or rooflight | 30m² | £23,000 to £30,000 | 4 to 6 weeks |
| Rear dormer | 30m² | £40,000 to £60,000 | 10 to 12 weeks |
| L-shaped dormer* | 30m² | £40,000 to £60,000 | 10 to 12 weeks |
| Hip to gable | 30m² | £50,000 to £70,000 | 10 to 14 weeks |
| Hip to gable with rear dormer* | 30m² | £50,000 to £70,000 | 10 to 14 weeks |
| Mansard | 30m² | £50,000 to £80,000 | 12 to 16 weeks |
| Bungalow | 50m² | £63,000 to £88,000 | 10 to 12 weeks |
Ranges, not quotes. Build cost only: excludes VAT, professional fees, planning and building control. Last updated September 2026.
* Estimated from the per-m² rate of a related type: no established source prices it separately.
Sources: Checkatrade, Loft conversion cost: 2026 UK price guide (March 2026 (table), August 2026 (breakdown)); Homebuilding & Renovating, Loft conversion costs (Tim Phillips, quantity surveyor) (29 July 2026).
What pushes the cost up or down
- Type of conversion. Rooflight conversions leave the roof shape alone. Dormers, hip-to-gable and mansard conversions add structure, steelwork and roofing.
- Size. More floor area means more materials and labour, although larger projects can cost less per square metre.
- Roof structure. Older roofs can need more strengthening than expected, and modern trussed roofs need more structural work to open up.
- Bathrooms and fittings. An en-suite, bespoke storage and extra windows all add to the budget.
- Finish. Homebuilding & Renovating's per-m² figures rise by about 30% from a basic to a mid-range finish, and by about 75% for premium.
- Location. Homebuilding & Renovating says projects in the South East and London can cost 15% to 30% more than comparable projects elsewhere.
- Fees. Drawings, structural calculations, planning, building control and party wall agreements are separate from the build.
How we calculate this
This calculator is based on our research into published cost guides. It is a starting point for your budget, not a quote. We review the figures regularly and adjust them as new information comes in.
- Build cost by type. We start from Checkatrade's UK cost ranges for a typical 30m² loft conversion (50m² for bungalows), last updated March 2026.
- Location. For London and the South East, including Berkshire, we raise the low end by 15% and the high end by 30%, following Homebuilding & Renovating. As a check, the results land close to Homebuilding & Renovating's own published ranges for each type.
- Size. Costs are scaled by floor area, which is how Checkatrade suggests estimating lofts larger or smaller than 30m².
- Finish. Mid-range and high-end finishes multiply the build cost by 1.3 and 1.75, from the ratio of Homebuilding & Renovating's per-m² bands.
- Extras and fees. From Checkatrade's cost guides, Homebuilding & Renovating, GOV.UK planning fees and, for Berkshire, each council's published building control charges.
- VAT. Build figures are treated as excluding VAT, and 20% is added to the build, extras and professional fees when the VAT box is ticked.
- Where the money goes. The breakdown uses the proportions of Checkatrade's example breakdown for a 30m² dormer (August 2026), so it is indicative for other types.
No established source prices L-shaped dormers or hip-to-gable with a rear dormer separately, so the calculator uses the per-m² rate of the closest type. The floor areas behind the size options (25m², 30m², 40m² and 45m²) are our assumptions: use the floor area slider if you know yours.
Last updated 22 September 2026.
Sources
- Checkatrade, Loft conversion cost: 2026 UK price guide (March 2026 (table), August 2026 (breakdown))
- Checkatrade, How long does a loft conversion take? (August 2026)
- Homebuilding & Renovating, Loft conversion costs (Tim Phillips, quantity surveyor) (29 July 2026)
- GOV.UK, Planning fees: annual indexation from 1 April 2026 (1 April 2026)
- Checkatrade, Cost to add an ensuite bathroom (June 2026)
- Checkatrade, Balcony installation cost (Juliet balcony) (September 2026)
- Checkatrade, All you need to know about Velux windows (September 2024)
- Checkatrade, Fitted wardrobes cost (March 2025)
- Checkatrade, Underfloor heating cost (March 2026)
- Checkatrade, Party wall surveyor cost (June 2026)
- HMRC, Buildings and construction (VAT Notice 708)
- Nationwide, Bedrooms, bathrooms and extensions: what adds value to your home in 2023?
- Local house prices: HM Land Registry Price Paid Data. Building control fees: each Berkshire council's published charges.
Frequently asked questions
Do loft conversion prices include VAT?
Not in the build costs we show. Work to alter or extend an existing home is generally charged VAT at the standard 20% rate (HMRC VAT Notice 708), so the calculator adds it to the total project cost by default. A builder who is not VAT registered will not charge VAT, so untick the VAT box if that applies to your quote.
Do the prices include planning and building control?
The build cost does not. The total project cost adds the fees you tick: a planning application (£548 in England from April 2026) or a lawful development certificate (£274), architectural drawings, a structural engineer, a party wall surveyor and building control. For Berkshire postcodes we use the local council's published building control fee.
What is the cheapest type of loft conversion?
A Velux or rooflight conversion is usually cheapest, because it adds windows to the existing roof slope without changing the roof structure. It needs enough head height in the loft already.
How much does a loft conversion cost per square metre?
Checkatrade gives typical UK averages of about £920 per m² for a Velux conversion, £1,670 for a dormer, £2,000 for hip-to-gable and £2,170 for mansard (March 2026). Homebuilding & Renovating gives £1,800 to £4,000+ per m² excluding VAT depending on the finish (July 2026). Figures vary with the size, structure and location.
Does a loft conversion add value?
It can. Nationwide's 2023 research found that adding a bedroom took about 10% to 12% onto the value of a three-bedroom house, and a loft conversion or extension adding a large double bedroom and bathroom could add as much as 25%. These are national averages, not a valuation of your home.
Next steps
For costs across Berkshire, including costs by town and house type, read our loft conversion cost guide. To compare types, see dormer, hip-to-gable, mansard and Velux conversions.
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