CIL (Community Infrastructure Levy)
CIL is a per-square-metre charge on new floorspace, paid to the borough at commencement of development. Rates vary widely: Westminster £450–£600/m², K&C £400–£550/m², Wandsworth £250–£500/m², outer boroughs £40–£200/m². Owner-occupier extension to a principal residence over 100m² qualifies for the Self Build Exemption — but only if Form 9 is submitted and accepted before commencement, then occupancy continues 3 years post-completion.
Self-build exemption process
Step 1: Apply for planning. Step 2: Receive consent. Step 3: Submit CIL Form 1 (assumption of liability) and CIL Form 7 (notice of chargeable development). Step 4: Submit Form 9 (Self-Build Annex/Extension Exemption Claim) BEFORE works commence. Step 5: Submit Form 6 (commencement notice). Step 6: Complete works, occupy as principal residence for 3+ years. Failure to file Form 9 before commencement = no exemption, full CIL due. Failure to occupy 3 years = retrospective CIL clawback.
Section 106
Section 106 (S106) is negotiated planning obligation — affordable housing contributions, transport mitigation, public realm. Triggered on larger development creating new dwellings (e.g., subdividing a house, building a new dwelling), not on extensions to existing dwellings. Residential extensions almost never trigger S106 unless they create a new dwelling unit (e.g., extension + change of use to flat conversion).
