What is CIL and when does it apply?
Community Infrastructure Levy is a fixed-rate charge per square metre of new floorspace, set by each borough in its CIL Charging Schedule. London also has a Mayoral CIL (currently £35–£90/m² depending on zone, funding Crossrail and Elizabeth Line). Borough CIL ranges widely — Wandsworth £575/m² in zone 1 down to nil in low-value areas; Camden £200–£350/m²; Westminster £350–£550/m². CIL applies to new floorspace created by planning permission, calculated on Gross Internal Area added. Critical exemption: householder extensions on a single dwelling are exempt where the addition is less than 100m² gross floor area AND the dwelling was already residential and stays residential. Most London single-storey rear extensions, loft conversions and side returns fall under 100m² and are exempt. Two-storey extensions above 100m² and full new dwellings pay CIL.
What is Section 106 and when does it apply?
Section 106 of the Town and Country Planning Act allows the local authority to require a legal agreement before granting planning permission. Common contents on residential schemes: affordable housing contribution or on-site provision (usually only on schemes of 10+ units or where policy triggers), education and health contributions, transport and travel plan, restriction on later subdivision, restriction on extension via PD ('Section 106 PD removal'). Section 106 rarely applies to single-house householder extensions but can apply to new dwelling creation (knock-down-rebuild, side garden development) or where a council uses it to remove PD rights on a new build. Section 106 is negotiated case-by-case and the financial contribution is on top of CIL — they are complementary not alternative.
How to calculate CIL liability and exemptions
Step 1: total gross internal area of new floorspace (extensions plus any new dwelling) minus existing floorspace being demolished and replaced (within 7 years). Step 2: check if householder exemption applies — same residential dwelling AND <100m² net new GIA. If yes, submit Form 1 (Assumption of Liability) and Form 2 (Householder Exemption Claim) before commencement — exemption granted, no payment. Step 3: if exemption doesn't apply, calculate liability = GIA × (Mayoral CIL rate + Borough CIL rate) × inflation index. Step 4: submit Commencement Notice before starting work; payment is typically due 60 days after commencement on small schemes or in installments on larger schemes. Step 5: comply with all CIL forms — failure to submit Commencement Notice triggers full payment immediately plus 20% surcharge. The CIL administrative process is unforgiving and is the source of many builder claims against architects who didn't manage it.
