What the toolkit is
Free (Excel download, GLA-published). Source: london.gov.uk Affordable Housing & Viability SPG (verified 2026-06-21).
Three Dragons-maintained spreadsheet; still the GLA reference toolkit for commuted-sum calculation across London boroughs.
Commuted-sum mechanic in the toolkit
The per-dwelling commuted sum equals build cost (BCIS-indexed) less the capitalised rent value at the assumed tenure mix. For social-rent shortfalls the capitalised value is lower (longer amortisation, regulated rent), so the commuted sum is higher.
Tenure-split inputs
Default GLA tenure mix is 60% social rent or London Affordable Rent plus 40% shared ownership. Boroughs may adopt local variations; RBKC and Camden carry local-housing-needs evidence that shifts the mix toward social rent.
Named limitations versus full residual
The toolkit isolates the AH commuted sum; it is not a full residual viability appraisal. For viability cases challenging the policy-compliant tariff overall, the toolkit feeds the AH line of a wider Argus Developer or Aprao model.