Cellar conversion cost per m2 (10 figures)
The median published figure in the Basement Conversion Cost Index is 2,250 GBP across 10 figures from 8 sources' own pages (range 1,080 to 6,000 GBP), read 12 October 2026.
- 41,080 to 1,900
- 31,900 to 2,720
- 22,720 to 3,540
- 3,540 to 4,360
- 4,360 to 5,180
- 15,180 to 6,000
Bands
| Bucket | Value |
|---|---|
| 1,080 to 1,900 | 4 figures |
| 1,900 to 2,720 | 3 figures |
| 2,720 to 3,540 | 2 figures |
| 3,540 to 4,360 | 0 figures |
| 4,360 to 5,180 | 0 figures |
| 5,180 to 6,000 | 1 figure |
8 of the 26 named sources read in full publish a figure; 18 publish none, read 12 October 2026.
The median is up 550 GBP since 26 September 2026 (was 1,700 GBP across 8).
By region and depth
| Breakdown | Median | Range | Sources |
|---|---|---|---|
| regionLondon | 4,250GBP | 2,500 to 6,000GBP | 2 |
| regionCentral London | 2,500GBP | 2,500 to 2,500GBP | 1 |
| depthcellar fit-out | 1,200GBP | 1,080 to 1,400GBP | 4 |
| depthnew dig | 3,000GBP | 2,000 to 3,000GBP | 3 |
Every figure, with its source
One row per published figure: the figure as the source's own page publishes it, the page, and the day it was read. A figure that is not on a page we read is not on this one.
| Source | Figure | Breakdown | Read |
|---|---|---|---|
| Architecture for London, basement costs in London | 6,000GBP | London | 2026-10-11 |
| houseUP, basement conversion cost | 3,000GBP | new dig | 2026-10-11 |
| Warvena Construction, basement conversion cost | 3,000GBP | new dig | 2026-10-12 |
| ALTER Architects, architect's cost guide | 2,500GBP | Central London | 2026-10-11 |
| Alstruct, London basement conversion guide | 2,500GBP | London | 2026-10-12 |
| Alstruct, London basement conversion guide | 2,000GBP | new dig | 2026-10-12 |
| Dry Basements UK, damp-proofed conversion pricing | 1,400GBP | cellar fit-out | 2026-10-11 |
| houseUP, basement conversion cost | 1,200GBP | cellar fit-out | 2026-10-11 |
| Renovat (Manchester), are cellar conversions worth it | 1,200GBP | cellar fit-out | 2026-10-12 |
| Beams, basement and cellar conversion cost | 1,080GBP | cellar fit-out | 2026-10-11 |
The sentence each figure was read from (10)
- Architecture for London, basement costs in London, read 2026-10-11:
As a headline summary, basement costs in London start at around £6,000 per square metre plus VAT.
- houseUP, basement conversion cost, read 2026-10-11:
Converting an existing cellar costs £1,200 to £2,250 per square metre, so a typical project runs about £25,000 to £75,000. Excavating a brand-new basement is far more, at £3,000 to £4,650 per square metre, and a larger new basement can exceed £110,000.
- Warvena Construction, basement conversion cost, read 2026-10-12:
This runs £3,000 to £5,000+ per square metre, with a typical 30m² new-dig project landing around £90,000 to £150,000 before professional fees — and considerably more in London, where access constraints and labor costs push figures toward £6,000–£10,000/m².
- ALTER Architects, architect's cost guide, read 2026-10-11:
Central London:** £2,500-£4,000/m²
- Alstruct, London basement conversion guide, read 2026-10-12:
London property values in desirable areas range from £8,000 to £15,000+ per square metre. Creating new basement space costs £2,500–£6,500 per square metre, usually well below the value it adds.
- Alstruct, London basement conversion guide, read 2026-10-12:
London property values in desirable areas range from £8,000 to £15,000+ per square metre. Creating new basement space costs £2,000–£4,000 per square metre — a fraction of the value it adds.
- Dry Basements UK, damp-proofed conversion pricing, read 2026-10-11:
Habitable fit-out — 50m² mixed use @ £1,400-1,800/m² £23,000 — £45,000
- houseUP, basement conversion cost, read 2026-10-11:
Basement conversions are generally more cost-effective than traditional extensions, with costs ranging from £1200 to £2,250 per square metre for existing cellars.
- Renovat (Manchester), are cellar conversions worth it, read 2026-10-12:
Converting an existing cellar typically runs from around £1,200 to £2,250 per square metre in 2026, with most usable fit outs landing roughly £25,000 to £60,000.
- Beams, basement and cellar conversion cost, read 2026-10-11:
Planning and budgetingBasement conversionsCosts Converting a cellar you already have costs £1,080 to £1,680 per square metre including VAT, or around £32,400 to £50,400 for a 30 square metre cellar.
Read in full, publishes no figure
18 named sources were read in full on the dates shown and print no usable figure. They are counted, never filled in.
- LCCL Construction, London basement cost, read 2026-10-11: every candidate on the page was refused
- TrustBuilt Projects, London price breakdown, read 2026-10-11: every candidate on the page was refused
- Plans Made Easy, cellar conversion cost, read 2026-10-12: every candidate on the page was refused
- AllDamp, cost to tank a cellar, read 2026-10-12: read in full and no accepted figure on the page
- LABS Basements and Damp Proofing, cellar conversion cost, read 2026-09-11: every candidate on the page was refused
- YDC Basements, basement conversions, read 2026-10-12: read in full and no accepted figure on the page
- JRM Cellar Conversions, read 2026-09-12: read in full and no accepted figure on the page
- MyBuilder, cellar conversion price guide, read 2026-10-12: every candidate on the page was refused
- MyBuilder, cellar tanking cost, read 2026-10-12: every candidate on the page was refused
- MyJobQuote, basement conversion cost, read 2026-10-11: every candidate on the page was refused
- BookaBuilderUK, basement conversion cost, read 2026-09-11: every candidate on the page was refused
- TopTradespeople, basement conversion cost, read 2026-10-12: every candidate on the page was refused
- Homebuilding & Renovating, basement conversion costs, read 2026-10-11: every candidate on the page was refused
- The Page, London basement conversion cost, read 2026-10-11: every candidate on the page was refused
- Basements North West, cost FAQ, read 2026-10-11: read in full and no accepted figure on the page
- Bosaco (Nottingham), basement conversions, read 2026-09-12: read in full and no accepted figure on the page
- Newcastle Cellar Conversion, read 2026-09-12: read in full and no accepted figure on the page
- the basement contractor universe, read 2026-10-12: read in full and no accepted figure on the page
Methodology
Every row is a verbatim sentence read from the named source's own page through our fetch service on the date shown, hash-pinned to the stored read (the sha of the page's visible text; the raw page is mirrored to R2).
8 of the 26 named sources that could be read published a usable figure; 18 published none and are recorded as not published, never filled in; 1 could not be read (blocked, dead or unreachable) and count nowhere.
The headline figures (median, minimum, maximum, quartiles) are over ONE figure per source for each breakdown it prices, in GBP (a source's median where its page yielded several accepted sentences for one breakdown), so a page that gives figures for several breakdowns gives one for each and N counts 10 figures, never sentences; 208 candidate sentence(s) were refused by a reviewer with the reason recorded.
The fuller derivation this dataset is chartered to grow into (not yet what the figures above are; they are each source's own published figure as read): the per-square-metre rate per depth of work - fit-out of an existing cellar, lowering the floor, underpinning, a new dig - per region, from every contractor that publishes one, with the spread between them; the ratio of the same contractor's London rate to its regional rate; the waterproofing share of the total, which the reads put at 15-20% but which nobody has measured across publishers; and the multiple a basement costs over the garage index for the same added floor area, which is the comparison the buyer is actually making.
And the distribution of the HEADROOM at which each firm says underpinning or floor lowering starts - the threshold that decides whether the buyer has a fit-out or an excavation, stated by some firms in their own words and published as a market figure by nobody.
208 candidate sentences on these pages were refused by a reviewer; each refusal and its reason is recorded in the review ledger.
Sources (27)
- houseUP, basement conversion cost(the publisher's own terms; quoted for reporting and comment)
- LCCL Construction, London basement cost(the publisher's own terms; quoted for reporting and comment)
- Warvena Construction, basement conversion cost(the publisher's own terms; quoted for reporting and comment)
- ALTER Architects, architect's cost guide(the publisher's own terms; quoted for reporting and comment)
- Beams, basement and cellar conversion cost(the publisher's own terms; quoted for reporting and comment)
- Dry Basements UK, damp-proofed conversion pricing(the publisher's own terms; quoted for reporting and comment)
- TrustBuilt Projects, London price breakdown(the publisher's own terms; quoted for reporting and comment)
- Architecture for London, basement costs in London(the publisher's own terms; quoted for reporting and comment)
- Alstruct, London basement conversion guide(the publisher's own terms; quoted for reporting and comment)
- Plans Made Easy, cellar conversion cost(the publisher's own terms; quoted for reporting and comment)
- AllDamp, cost to tank a cellar(the publisher's own terms; quoted for reporting and comment)
- LABS Basements and Damp Proofing, cellar conversion cost(the publisher's own terms; quoted for reporting and comment)
- Renovat (Manchester), are cellar conversions worth it(the publisher's own terms; quoted for reporting and comment)
- YDC Basements, basement conversions(the publisher's own terms; quoted for reporting and comment)
- JRM Cellar Conversions(the publisher's own terms; quoted for reporting and comment)
- MyBuilder, cellar conversion price guide(the publisher's own terms; quoted for reporting and comment)
- MyBuilder, cellar tanking cost(the publisher's own terms; quoted for reporting and comment)
- MyJobQuote, basement conversion cost(the publisher's own terms; quoted for reporting and comment)
- BookaBuilderUK, basement conversion cost(the publisher's own terms; quoted for reporting and comment)
- TopTradespeople, basement conversion cost(the publisher's own terms; quoted for reporting and comment)
- Homebuilding & Renovating, basement conversion costs(the publisher's own terms; quoted for reporting and comment)
- The Page, London basement conversion cost(the publisher's own terms; quoted for reporting and comment)
- Basements North West, cost FAQ(the publisher's own terms; quoted for reporting and comment)
- Bosaco (Nottingham), basement conversions(the publisher's own terms; quoted for reporting and comment)
- Newcastle Cellar Conversion(the publisher's own terms; quoted for reporting and comment)
- Checkatrade, cellar conversion cost guide(the publisher's own terms; quoted for reporting and comment)
- the basement contractor universe(the publisher's own terms; quoted for reporting and comment)
Cite or embed this figure
Cite or embed this figure
The median published figure in the Basement Conversion Cost Index is 2,250 GBP across 10 figures from 8 sources' own pages (range 1,080 to 6,000 GBP), read 12 October 2026.
8 of the 26 named sources read in full publish a figure; 18 publish none, read 12 October 2026.
Basement Conversion Cost Index, region London: median 4,250 GBP across 2 sources, read 12 October 2026.
Basement Conversion Cost Index, region Central London: median 2,500 GBP across 1 source, read 12 October 2026.
Basement Conversion Cost Index, depth cellar fit-out: median 1,200 GBP across 4 sources, read 12 October 2026.
Basement Conversion Cost Index, depth new dig: median 3,000 GBP across 3 sources, read 12 October 2026.
The median is up 550 GBP since 26 September 2026 (was 1,700 GBP across 8).
Cite as: "Roofing Quotes UK Basement Conversion Cost Index", updated 2026-10-23, https://roofingquotesuk.com/datasets/basement-conversion-cost-index/.
Download the CSV (10 rows, computed 2026-10-23)