CIL and the exemptions: the levy you can lose by missing a form

The Community Infrastructure Levy (CIL) is a charge on new floorspace, levied per square metre by the council — and in London, a second time by the Mayor — to fund infrastructure. For most homeowners it amounts to nothing, because small extensions are exempt. But CIL has a vicious edge: where an exemption has to be claimed, missing the paperwork or starting work too early doesn't just delay it — it loses the exemption entirely and adds a surcharge on top. People pay four- and five-figure CIL bills they never owed, purely on procedure.

This page explains when CIL is actually charged, the exemptions that cover most domestic work, the precise sequence you must follow to keep an exemption, and the London layer — the Mayoral CIL, with its current 2026 rates. It deals with England, and the Mayoral CIL applies across Greater London.

When CIL is — and isn't — charged

• Development of less than 100m² of new internal floorspace is exempt under the "minor development" exemption — so a typical single-storey extension is usually outside CIL automatically.
• But a new dwelling is liable even if it's under 100m² — the minor-development exemption doesn't apply where a new home is created.
• Extensions of 100m² or more of new floorspace are liable — unless you claim the residential extension exemption.
• A residential annex in the grounds of your own home, and a self-build dwelling you build and occupy, are liable in principle but exempt if you claim — under the self-build exemptions.
• Buildings you enter and use are counted; converting existing floorspace that has been in lawful use generally isn't new floorspace for CIL.

The sequence that keeps your exemption

For anything where you're claiming a CIL exemption or relief, the order of the forms matters more than almost anything else in this guide. Get it in this order, before any work starts:

• Assume liability. Submit the Assumption of Liability form (this is required for the annex and self-build exemptions; it isn't needed for the self-build residential extension exemption, but submitting it does no harm).
• Claim the exemption. Submit the relevant exemption form and wait for the council to grant it in writing. Do not start on the strength of having applied — you need the decision.
• Submit a Commencement Notice — before you begin — for development that needs one, which includes new dwellings and annexes. (A development consisting wholly of a residential extension does not need a Commencement Notice, but you still must have the exemption granted before you start.)
• Then commence. Starting work before the exemption is granted, or before a required Commencement Notice is submitted, loses the exemption and triggers a surcharge.

Two more traps worth stating plainly: retrospective permission for work already built generally cannot claim a CIL exemption, so building first and regularising later can convert an exempt project into a liable one; and failing to submit a Commencement Notice where one is required is itself a surcharge, separate from losing the relief.

The London layer: Mayoral CIL (MCIL2)

In Greater London, the Mayor charges a second CIL — currently the MCIL2 schedule — on top of the borough's own CIL, to help fund the Elizabeth line. It's banded by borough and indexed every year against the RICS construction index. For planning permissions granted in calendar year 2026, the rates per square metre of liable new floorspace are:

How this connects to your Planning Permission Checker report

Your borough sets its own CIL rates and exemption procedures, and the Mayoral band depends on which borough you're in — both things the Planning Permission Checker report's borough context helps you place. The report doesn't file your CIL forms. For a new dwelling, an annex, or a larger extension where the exemption paperwork and timing carry real money, enquiries route to Hampstead Chartered Surveyors, an RICS-regulated practice (Firm Reg. 923064).