What Is a CIL Charge and When Does It Apply to Your Extension?
The Community Infrastructure Levy (CIL) is a charge that some local authorities apply to new developments — including residential extensions and conversions — to fund local infrastructure. Not all councils charge CIL, and the rates vary significantly where it does apply. In some London boroughs, CIL charges on a moderate extension can reach several thousand pounds. Understanding when CIL applies — and when you can claim an exemption — is part of the financial planning for any significant building project.
When Does CIL Apply?
CIL applies where: the local planning authority has adopted a CIL Charging Schedule (check your council's website — not all have); the planning permission creates a new dwelling, or creates additional internal floor area above certain thresholds; the works require planning permission (CIL does not apply to Permitted Development works, though some councils have additional CIL charges). The trigger for CIL on residential extensions is typically the creation of more than 100m² of gross internal area of new build (chargeable floor space), or where a new dwelling is created.
An extension to a house that remains a single dwelling and adds less than 100m² typically does not attract CIL. However, a loft conversion or extension that crosses this threshold, or any conversion that creates a second dwelling (self-contained flat or annexe), may attract a CIL charge — especially in boroughs with relatively high rates such as those in central London.
The Self-Build Exemption
The most important exemption for homeowners is the Self-Build Exemption. Where the homeowner is building or commissioning a new dwelling or extension to their own principal home (i.e., where they live or intend to live), and they occupy the dwelling as their principal residence for at least three years after the works, they can claim a full exemption from CIL. The exemption must be claimed before work starts — you cannot claim it retrospectively. The process involves submitting a specific form (Form 7: Exemption Claim — Self Build Dwelling) to the planning authority before work commences.
When to Check
Check with your local council's planning or CIL team early in the planning process — ideally before submitting your planning application. They can confirm whether CIL applies in your area, what the rate is, whether your project is chargeable, and what exemptions might be available. The consequences of missing a CIL charge can include a surcharge and loss of appeal rights, so early awareness is essential.
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