How we calibrate every number on this site.

Every cost figure on this site is calibrated on published UK construction data. Nothing is invented. Nothing is “based on a survey we did” without telling you what the survey was. This page is the audit trail.

Primary data sources

Three published datasets anchor every cost band on the calculator, the Snapshot AI estimator, and the editorial guides:

• BCIS (Building Cost Information Service) Q1 2026. The RICS-administered cost dataset that every chartered quantity surveyor in the UK uses. We use the Composite Building Costs index for residential renovation, the All-In Tender Price Index for regional uplift, and the Resource Cost of Materials index for the materials line.
• FMB (Federation of Master Builders) Master Builder Survey 2026. The annual UK trade day-rate survey covering bricklayer, carpenter, electrician, plumber, plasterer, painter-decorator, roofer and labourer. Sampled across all 9 English regions, Wales, Scotland, and Northern Ireland.
• ONS (Office for National Statistics) Construction Output Price Indices, latest quarterly release. Used to roll forward BCIS Q1 figures within-quarter and to validate the labour-cost component of FMB rates against the official CPI/RPI inflation prints.

Regional uplift factors

The calculator and the Snapshot AI both apply a regional cost factor before final output. The factors below are BCIS Q1 2026 derived, restated for homeowner readability:

• Prime Central London (W1, SW1, SW3, SW7, NW8, W8): +35% to +45%
• Inner London (E1–E20, N1, SE1–SE10, W2–W14, SW2–SW20): +20% to +30%
• Outer London + South East: +10% to +20%
• South West, East England: 0% to +10%
• Midlands, North West: −5% to +5%
• Yorkshire, North East: −10% to −15%
• Wales, Scotland: −5% to +5% (Edinburgh +10%)
• Northern Ireland: −15% to −25%

VAT treatment

We apply the 20% standard UK VAT rate by default. Three concessions are explicitly modelled in the calculator and flagged in the quote analyser:

• 0% on energy-saving materials (solar PV, heat pumps, insulation, extended to March 2027).
• 5% reduced rate for conversions from non-residential to residential.
• 5% reduced rate for properties empty 2+ years; 0% for properties empty 10+ years.

Snapshot AI calibration

The Snapshot AI estimator uses Anthropic’s Claude Sonnet 4.5 vision model. The model receives, in this order:

1. A system prompt explaining the UK 2026 cost landscape, the four scope tiers, and the dating tell-tales for UK housing stock (Artex ceilings, melamine units, avocado bathrooms, single-glazed timber sashes, pebbledash render, etc.).
2. The BCIS-derived calculator benchmark block (low/typical/high bands across four size brackets for the user’s region and spec) as a hard floor the model must respect.
3. Anti-staging guidance — the model is explicitly told that UK estate-agent photos (Foxtons, Knight Frank, Savills, Hamptons, Strutt & Parker) are professionally staged and that it should anchor on age and layout, not photo polish.
4. The user’s photo(s) or, in URL mode, every photo we can extract from the listing’s gallery (up to 8 images), plus the listing title, asking price and a 1,500-character excerpt of the description.

The model is constrained to call a structured JSON tool, not free-text. The schema demands a low/typical/high cost band, a verdict bucket, 4–6 line items with rationale, a caveats list, and a confidence score 0–1.

We tested this calibration against a chartered UK quantity surveyor on a real 1980s Inner-London semi — full methodology and results in our AI cost estimation guide.

Quote AI calibration

The quote analyser uses the same Claude Sonnet 4.5 model on PDF and image inputs. The prompt is calibrated on the eight-point UK builder’s quote checklist (itemisation, contingency, VAT treatment, provisional sums, payment schedule, programme, contract terms, insurance certificates). Output is structured against a fixed schema that scores the quote against each checklist item and produces a plain-English summary.

Review cadence

We refresh the cost calibration quarterly against the latest BCIS print, FMB day-rate update, and ONS COPI release. Editorial guides carry a “Last reviewed” date in the header. Anything more than 12 months stale is flagged in the article and prioritised for rewrite.

What we don’t do

• We don’t sell leads to contractors. There are no affiliate links to builders, architects or surveyors anywhere on the site.
• We don’t train any AI model on user submissions. Snapshot and quote uploads go to Anthropic under their published data-processing terms and are not used for training under our agreement.
• We don’t store uploaded photos or quote PDFs beyond 24 months. Deletion on request, GDPR-compliant — full detail in the privacy policy.
• We don’t accept sponsored content. Nothing on the site is paid placement.

Corrections and challenges

We get things wrong. When that happens we want to know. Email admin@untangle.ie with the page URL, the figure you believe is wrong, and your source for the alternative. Confirmed corrections ship within one working week and the page’s “Last reviewed” stamp updates.

This methodology page is maintained alongside the calculator code. Last reviewed 21 May 2026 against BCIS Q1 2026 + FMB 2026 + ONS COPI Q1 2026. Next scheduled review: October 2026.