Loft Conversion Cost Calculator: An Indicative London Budget
Last reviewed: 6 November 2026
Enter the loft floor area you could gain and the finish level. The calculator applies the London figures published in our loft conversion cost guide and shows an indicative range. Every figure includes VAT at 20%. It is a budgeting tool, not a quote.
Loft conversion cost estimator
Indicative only — including VAT, before professional fees and contingency. Confirm a written scope and itemised quote.
How to use this estimate: enter the floor area you would gain in the loft — a typical London loft conversion is 20–35 m². The London figures in our guide are published by size, and the calculator reproduces them exactly: 20 m² £39,000 – £60,000, 30 m² £58,500 – £90,000 and 40 m² £78,000 – £120,000, all including VAT at 20%.
What this range includes and excludes
- Includes VAT at 20%. Our loft conversion cost guide states its figures including VAT, so the calculator does too — unlike our extension calculator, which works before VAT.
- London allowance: +30% on the lower rate and +50% on the upper rate, which is the 30–50% London premium our guide describes. The calculator always applies it.
- Finish: standard finishes are included; premium finishes use the guide's published £10,000 – £25,000 allowance, added to both ends of the range.
- Excludes: professional fees, planning application and party wall costs, building control, contingency, and a bathroom or en-suite (the guide puts that at an extra £5,000 – £12,000 including VAT).
- Not a price: the result is rounded to the nearest £1,000 and is a budgeting range, not a quotation.
Rates used in this calculator
| Conversion size | Published London low (incl. VAT) | Published London high (incl. VAT) | Lower rate per m² | Upper rate per m² |
|---|---|---|---|---|
| 20 m² | £39,000 | £60,000 | £1,950 | £3,000 |
| 30 m² | £58,500 | £90,000 | £1,950 | £3,000 |
| 40 m² | £78,000 | £120,000 | £1,950 | £3,000 |
The guide's London table is not broken down by conversion type. It corresponds to +30% on the guide's minimum rate of £1,500 per m² and +50% on its average rate of £2,000 per m². That is the model this calculator uses, so its output matches the guide's London rows rather than deriving a new type-by-type London figure.
What the conversion type changes nationally
| Conversion type | Published national rate per m² (incl. VAT) | Published example | Published national total (incl. VAT) |
|---|---|---|---|
| Rooflight (roof windows only) | £1,500 – £2,000 | 20 m² | £30,000 – £40,000 |
| Dormer | £1,800 – £2,500 | 30 m² | £54,000 – £75,000 |
| Hip-to-gable | £2,000 – £2,800 | 35 m² | £70,000 – £98,000 |
| Mansard | £2,200 – £3,000 | 40 m² | £88,000 – £120,000 |
| Room-in-roof (shell only) | £1,200 – £1,800 | 20 m² | £24,000 – £36,000 |
Worked examples from our published guide
The figures below are the guide's own published rows, recalculated from the rates above.
| Published example | Our guide says | Calculator's rates give |
|---|---|---|
| London, 20 m² | £39,000 – £60,000 | £39,000 – £60,000 |
| London, 30 m² | £58,500 – £90,000 | £58,500 – £90,000 |
| London, 40 m² | £78,000 – £120,000 | £78,000 – £120,000 |
Frequently asked questions
How much does a loft conversion cost per m² in the UK?
Our loft conversion cost guide uses £1,500 to £2,500 per m² including VAT at 20% for a loft conversion nationally, with the minimum, average and maximum columns at £1,500, £2,000 and £2,500. In London the guide adds a 30% to 50% premium over the national average, which is what this calculator applies.
Does this calculator include VAT?
Yes. Every figure includes VAT at 20%, because that is how our loft conversion cost guide states its prices. It is before professional fees, planning and party wall costs, building control and contingency.
Why does the calculator not ask for the conversion type?
Our loft guide publishes its London figures by floor area rather than by conversion type: 20 m² £39,000 – £60,000, 30 m² £58,500 – £90,000 and 40 m² £78,000 – £120,000. The calculator reproduces those London figures exactly. The same guide's per-type tables are national construction rates, shown below for comparison; a bigger conversion type usually means a bigger footprint, which is the input that moves the London number.
What does a bathroom or en-suite add?
Our loft conversion cost guide puts a bathroom or en-suite at an additional £5,000 to £12,000 including VAT, depending on the plumbing route and soil-pipe access. That is on top of the range this calculator shows.
Is this a quote?
No. It is an indicative range from published rates, with no address, drawings or survey involved. The roof structure, access, party wall position and finish decide the real number. The next step is a free survey and a written, itemised quotation.
Plan your London loft conversion
Discuss your roof, the options and next steps at a free survey.
Book a free survey →Prefer to read the workings? Read the full loft conversion cost guide.
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