Commuted-sum mechanic
The commuted sum applies to the shortfall between the LPA's policy target (e.g. Birmingham TP30 at 35%) and the on-site percentage delivered. For a 50-unit scheme with a 35% target and 20% on-site delivery, the shortfall is 7.5 dwellings (typically rounded to 8). The commuted sum per dwelling is then build cost less capitalised rent value, derived from the GLA / Three Dragons toolkit for London boroughs [PPG Planning Obligations].
On-site versus off-site decision rule
Most adopted SPDs default to on-site delivery and treat off-site commuted sums as the exception. RBKC, Camden and Cambridge will accept off-site only where on-site delivery is demonstrably impractical (typically deliverability or RP nomination evidence). Smaller schemes below the LPA threshold (often 10 units for non-London authorities) attract a commuted sum by default rather than on-site delivery.
Grant interaction
Grant now comes through the Social and Affordable Homes Programme 2026-36, which succeeded the Affordable Homes Programme 2021-2026. In London the GLA runs it as the London Social and Affordable Homes Programme 2026-36, which opened for bids on 24 Feb 2026 and expects at least 60% of homes to be for social rent[London Social and Affordable Homes Programme 2026-36 (GLA)]. The treatment is unchanged: grant is generally additional rather than substitutive, and the GLA toolkit nets it against scheme viability rather than against the per-unit commuted sum. Grant-funded schemes must still meet the Aug 2026 NPPF viability position (Policy DM5) where the policy-compliant tariff is challenged.
Tenure-split assumption
The GLA toolkit defaults to a 60:40 social-rent or London-Affordable-Rent to shared-ownership split for London boroughs. Outside London the split follows the adopted SPD; Birmingham, Manchester and Leeds typically run with a higher shared-ownership share reflecting local market evidence.
RP nomination clause
Where on-site delivery is via a Registered Provider, the deed includes a nomination clause restricting first occupancy to households on the LPA's housing register. This binds successor RPs after stock transfer and is regularly tested at planning committee.