Levy liability at 100 square metres or one home

The CIL liability threshold is 100 m² of new floorspace, with one exception that swallows most householder projects: the minor development exemption does not apply where the development creates a dwelling. A 95 m² rear extension is exempt. A 38 m² studio flat in the garden is liable from the first square metre. That asymmetry sits in regulation 42 of the Community Infrastructure Levy Regulations 2010, and it explains most of the surprise demand notices that land on homeowners.

How the liability test works

The levy is charged on a chargeable development, meaning development for which planning permission is granted in an area with an adopted charging schedule. Where development is carried out under a general consent such as permitted development, and the charging schedule would catch it, a notice of chargeable development has to be served instead.

Two questions then decide liability. Does the development create 100 m² or more of new gross internal area? And does it create a dwelling, of any size?

The charging schedule itself is the other half of the answer. Rates are set by the charging authority, vary by use and often by geographical zone, and many schedules set no rate at all for some uses. Rates are indexed annually, so take the figures from the current published schedule rather than from an older copy or a consultant's note.

Measuring the floorspace

Gross internal area, which means measured to the internal face of the external walls and including internal walls, stairs, plant rooms and circulation. It is not the same as the net internal or net sales area used in agency work, and it is usually larger than people expect.

Three measurement points generate most disputes. Whether an integral garage counts, which depends on whether it is within the thermal envelope and how the authority treats it. Whether a mezzanine or a loft floor with permanent access counts, which it generally does once it is a usable storey. And how a part-single, part-two-storey extension is measured, which is simply the sum of the floors.

Ask the authority how it measures before you submit. The calculation appears on the liability notice, and arguing about it afterwards is harder than agreeing the method in advance.

Deducting existing lawful floorspace

The net chargeable area is the new build less the gross internal area of existing buildings that are being demolished or converted, but only where those buildings have been in lawful use. The test in the Regulations is lawful use for a continuous period of at least six months within the three years ending on the day planning permission first permits the chargeable development.

That six-months-in-three-years test is the one that costs people money. A barn empty for a decade, a shop vacant since before the acquisition, a house that has stood unoccupied through a probate dispute: in each case the floorspace may not be deductible, so the charge is calculated on the whole of the new build rather than on the net increase.

Evidence it properly and early. Utility bills, council tax or business rates records, tenancy agreements, trading accounts, dated photographs and witness statements all help. Where occupation is marginal, a short period of genuine lawful use before the application is determined can change the arithmetic substantially, which is a planning programme decision rather than an afterthought.

Reliefs and exemptions that may still apply

Even where liability arises, several routes remove or reduce it.

The self-build exemption applies to a dwelling built by or on behalf of the claimant and occupied as their sole or main residence for at least three years from completion. It is the most valuable exemption available to an individual and the easiest to lose.

The residential annex exemption applies to an annexe within the curtilage of the claimant's main residence. The residential extension exemption applies to an extension to the claimant's main residence that would otherwise be charged because it exceeds 100 m².

Social housing relief, charitable relief and, where the charging authority has adopted it, exceptional circumstances relief cover other cases.

Every one of these has to be claimed and granted before development begins. None of them is automatic, and none of them can be applied retrospectively.

The notices, and the consequence of missing them

This is a sequence, and the order is not negotiable.

1. Submit an assumption of liability form, naming who will pay.
2. Submit the exemption or relief claim, with its supporting evidence.
3. Wait for the authority to issue the liability notice, and for any exemption to be granted in writing.
4. Submit a commencement notice, stating the date development will begin, and receive acknowledgement.
5. Only then start work.
6. The authority issues a demand notice setting out the amount and the payment dates.

Starting work before the commencement notice is submitted loses any exemption or relief claimed, and the Regulations provide for a surcharge calculated as a proportion of the chargeable amount subject to a cap. The authority has no discretion to restore an exemption lost this way, and appeal rights on that point are narrow.

Development begins for levy purposes on the day a material operation is carried out. Digging a trench, laying a service duct or forming an access all count, so the sequence above has to be complete before the groundworker arrives rather than before the bricklayer does.

Illustrative example: a homeowner granted permission for a 42 m² self-contained annexe with its own kitchen, assessed by the authority as a new dwelling. The minor development exemption does not apply because a dwelling is created. The owner claims the residential annex exemption, which is granted, then starts footings two days before posting the commencement notice. The exemption falls away and a surcharge is added. The figures are illustrative rather than drawn from a real case, and the error is the common one.

Checking liability before work starts

Four checks, in this order.

Confirm whether the authority has an adopted charging schedule and what rate applies to your use and zone. Some authorities have none, in which case no levy arises at all.

Calculate the gross internal area of the new build and the deductible existing floorspace, with the six-months-in-three-years test applied to the latter.

Identify which exemption you are claiming and gather its evidence before the application is determined rather than after.

Diary the notice sequence against your start date, with a fortnight of slack.

Where the project is a single dwelling, the thresholds that change at the householder and minor application boundary are worth understanding at the same time, and we set those out in the note on where the single dwelling line falls. For larger schemes the levy sits alongside section 106 obligations, and the interaction is covered in our article on obligations and the levy at different scheme sizes.

One forward-looking point. The Levelling-up and Regeneration Act 2023 contains powers for an Infrastructure Levy intended to replace the existing levy, and those powers depend on regulations. Check the current position on legislation.gov.uk rather than assuming either regime has gone away.

A short Planning Statement that states the gross internal area, the existing floorspace and its use history, and the exemption being claimed makes the authority's calculation easier and reduces the chance of a liability notice you have to argue with. See PlanScope pricing if you want to draft one and have it reviewed before submission.

This is general guidance, not legal advice; check the position with your local planning authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.