MODEL 2026 10 03

How this estimate is produced

The calculator is a starting point for a feasibility conversation. Base rates are deliberately broad internal working allowances at a June 2025 price base. They have not been validated against Greyson & Cole completed projects or directly comparable public contracts. Confidence is capped at low.

Calculation and limits

A project-specific allowance is multiplied by affected area, internal regional and specification factors, complexity and access adjustments, and inflation. Site preliminaries and a separate risk allowance are then added. The subtype determines the baseline. Broad sector baselines receive a refurbishment, conversion or extension adjustment where applicable; these are included in the combined complexity factor. Work type also selects the observed inflation series. Figures are rounded outwards.

ONS Table 1 new-work indices change from 147.7 in June 2025 to 150.9 in June 2026. Repair and maintenance changes from 136.2 to 138.2. The corresponding ratio updates the assumed price base. After June 2026, a 0–6% annual compound scenario is an internal allowance, not an official forecast. An unknown tender date uses a disclosed 12-month scenario. Stale data widens the scenario and lowers confidence.

Materials statistics inform the context, not a second whole-project uplift: that would risk double counting. No public-contract value has been converted into a unit rate without a verified area, scope and price date. No paid proprietary cost data is used.

Unknowns and readiness

Unknown technical answers widen the cost range and appear in the follow-up list. An unknown area produces only a per-area allowance. No project total is invented. Readiness is a weighted preparation score: design 20 points, approvals 15, funding 15, site control 15, surveys 15, area 10, tender timing 5 and procurement preference 5. It is not a probability of planning permission or commercial success.

Fees are internal indicative coordination allowances. Programmes are broad internal scenarios. Specialist schemes, new basements, high-rise work and major infrastructure require professional review, including assessment of excluded packages.

Source register

RICS NRM 1

Methodology. Source updated: 2022-10. Last checked: 2026-10-03. Informs separation of works, preliminaries, risk, inflation and fees. No pricing supplied; no endorsement claimed. Source.

ONS Construction Output Price Indices

Official data snapshot. Source updated: 2026-08-13. Last checked: 2026-10-03. Table 1: June 2025 to June 2026. New work 147.7 to 150.9; repair and maintenance 136.2 to 138.2. 2015=100. Observed index ratio only, not a tender-price forecast. Source.

UK building materials statistics

Official context. Source updated: 2026-09-16. Last checked: 2026-10-03. July 2026 annual material movements: new housing 4.9%, other new work 7.2%, repairs 5.5%. Context only; not added again to ONS inflation. Source.

Planning Data

Public API. Source updated: Not supplied / unavailable. Last checked: At calculation. England only. Approximate postcode-point intersection; incomplete coverage. Not a site search or planning determination. Source.

Postcodes.io

Public location data. Source updated: Not supplied / unavailable. Last checked: At calculation. Region, local authority and postcode centroid. Contains OS data © Crown copyright and database right 2026; ONS data © Crown copyright and database right 2026. Northern Ireland licensing differs; no NI coordinates are reused. Source.

Contracts Finder

Context source not incorporated. Source updated: Not supplied / unavailable. Last checked: 2026-10-03. No comparable award with verified area, scope and price basis has been incorporated. No contract values used as £/m² evidence. Source.

Greyson & Cole working assumptions

Internal assumptions. Source updated: 2026-10-03. Last checked: 2026-10-03. Broad unvalidated rate allowances, location factors, fees, durations and risk factors. No completed-project dataset is available. Professional calibration required. Source.

Official snapshots were refreshed on 3 October 2026. Location data is requested at calculation time; the response may not disclose its own update date. No live index feed is claimed. Contains public sector information under the Open Government Licence v3.0 where applicable.