How the calculator builds the number
The engine starts with the BCIS small-residential £/sqm range for your chosen build-quality tier[BCIS Q2 2026], applies the regional location factor (the BCIS regional multiplier the QS profession uses), then layers nine cost lines: demolition, HSG264 asbestos survey and removal, foundations and ground investigation, build above slab, professional fees (architect plus structural engineer plus party-wall surveyor), planning application fees plus CIL plus Section 106, Building Control fees, temporary accommodation, and contingency.
All rates are anchored to publicly verifiable sources verified 21 June 2026. No quote-only vendor rates are inferred. Where a figure is a range rather than a point estimate (most of them), the calculator returns the full range.
What is excluded
Land cost is excluded. Demolition assumes the existing footprint is broadly equivalent to the replacement footprint (typical 1-for-1 KDR); a much larger replacement on a small existing footprint will under-state demolition cost. Soft landscaping, swimming pools, basements, and detached garages are excluded; tier rates assume conventional foundations and standard envelope detailing.
What it is not
This is not a quote and it does not bind a contractor. It is a defensible reference range to take into a Stage 2 conversation with your architect and into a self-build mortgage application. If your final tender comes in significantly outside the range, ask why; if it comes in inside, the calculator did its job.