Surveyor's note
This page sets out where the numbers come from and what the site does not pretend to know. The buyer is paying £350k-£800k on a building decision; transparency on method is the trust currency, not a glossy methodology page.
Where the £/m² bands come from
The bands are our own indicative scoping model, not figures lifted from a single published dataset. We start from a base build-cost per m² by spec tier (below) and adjust it by barn type, condition, planning route and region. The base rates reflect typical UK agricultural-to-residential conversion build costs; we inflate them to a Q2 2026 footing using ONS Construction Output Price Indices (April 2026 release). For an authoritative benchmark on a specific building, a chartered quantity surveyor can price it against RICS BCIS (a subscription cost database); we do not reproduce a specific BCIS release here.
The three spec tiers
- Modest, £1,800/m² base: standard kitchen, vinyl flooring, basic sanitaryware, simple finishes.
- Mid, £2,500/m² base: branded kitchen, engineered oak flooring, mid-tier sanitaryware, UFH throughout, integrated joinery.
- High, £3,400/m² base: bespoke kitchen, stone or natural-fibre floor, high-end sanitaryware, joinery throughout, exposed services as feature.
The multipliers
Barn-type, condition, planning-route and region multipliers are documented in src/data/calc-multipliers.ts. They are the model's own judgement factors, calibrated to typical build-cost differentials rather than a published index. Total multiplier = base × barn-type × condition × planning × region.
What the calculator does not do
- It does not give a fixed quote. It gives a band. Treat low-mid-high as the model's cautious, central and stretched case, not a statistical percentile from a measured dataset.
- It does not include land purchase, demolition, or any commercial-to-residential conversion (use VAT Notice 708 advice for those).
- It does not include equestrian or livery conversions; the Class Q regime does not cover them.
- It does not predict outcome on borderline Class Q applications; refusal-risk premium is a separate read.
Review
Reviewed for build by O. Wakefield-Smith, Digital Signet research, Q2 2026. RICS MRICS sign-off for the live site is being procured. The site will display the QS' name on this page and on every page footer once contracted.