The Community Infrastructure Levy, explained.

CIL is one of the easiest ways to lose thousands on a self-build or large extension. Here is how it works, and why you must follow the process to the letter, even when you are exempt.

The Community Infrastructure Levy, or CIL, is a charge that some councils place on new development to help fund local infrastructure. It catches a lot of self-builders and homeowners out, not because the charge itself is unfair, but because the procedure around it is strict and unforgiving. Get a form or a date wrong and you can face a surcharge, or lose an exemption you were fully entitled to.

What CIL is, and when it applies

CIL is charged per square metre of new floorspace, at rates set out in each council's charging schedule. Not every council charges it, and rates vary widely, so the first job is always to check your local authority.

• New dwellings are chargeable, whatever their size.
• Extensions and annexes are generally chargeable once they create 100 square metres or more of new floorspace.
• Minor works under 100 square metres that do not create a new dwelling are usually exempt.

Exemptions and reliefs

There are valuable exemptions, the most common being the self-build exemption for a home you are building to live in yourself. There are also exemptions for residential extensions and annexes, and relief for social housing and charities. The key point: none of these is automatic. You have to claim them, and follow the procedure exactly.

The process, step by step

1. Planning is granted and the council issues a Liability Notice.
2. You submit an Assumption of Liability form to confirm who will pay.
3. You claim your exemption or relief (for a self-build, this is the Self-Build Exemption Claim Part 1) and wait for it to be granted in writing. Do not assume it is in place.
4. You submit a Commencement Notice before any work begins, and make sure the council acknowledges it.
5. Only then do you start on site.
6. After completing a self-build, you submit the Self-Build Exemption Claim Part 2 with supporting evidence within six months of completion.
7. You keep the home as your main residence for three years, the clawback period.

Why the process matters, even with an exemption

This is the part people underestimate. An exemption is not "set and forget". If you do not follow each step, being entitled to the exemption will not save you.

• Miss the Commencement Notice before you start, and you face a surcharge of the lower of 20 per cent of the CIL that would have been due, or £2,500, even though you were exempt.
• Fail to assume liability, and the council can add surcharges and remove your right to pay in instalments, so the whole amount can fall due at once.
• Miss the six-month post-completion evidence, and the self-build exemption can be withdrawn, leaving the full CIL payable.
• Sell or let the home within the three-year clawback period, and the exemption is revoked. You must tell the council within 14 days and CIL becomes payable in full.

The risks to avoid, in short

• Assuming CIL does not apply, or that an exemption is automatic.
• Starting any work before the Commencement Notice is acknowledged.
• Forgetting to assume liability, or missing a form deadline.
• Selling or renting out within three years of completing a self-build.
• Not checking whether your council charges CIL, and at what rate, right at the start.

How we help

We flag CIL at the very beginning of a project, work alongside your architect or planning consultant, and make sure not a spade goes in the ground until liability is assumed, any exemption is granted, and the Commencement Notice is acknowledged. It is exactly the kind of detail that quietly protects you from a five-figure mistake.

CIL is complex and varies by council. This is general guidance for 2026, not formal advice. Always confirm the position with your local authority and take professional advice on your specific project.