The 100 square metre line in levy liability

The 100 square metre line in the Community Infrastructure Levy is narrower than most people assume. Regulation 42 of the Community Infrastructure Levy Regulations 2010 exempts minor development where the gross internal area of new build is less than 100 m², but the exemption is switched off the moment the development comprises one or more dwellings. A 58 m² one-bedroom house is fully chargeable. A 95 m² extension to an existing house is not chargeable at all.

That single carve-out causes more unexpected levy bills on small sites than any other part of the regulations. If you are building a plot, plan on paying unless a separate exemption applies, and claim it properly rather than hoping the threshold catches you.

The minor development exemption and what switches it off

Three conditions have to hold for regulation 42 to apply:

• The gross internal area of new build on the chargeable development is less than 100 m². Not 100 m² exactly. Below it.
• The development does not comprise one or more dwellings. A dwelling means a self-contained unit, so a studio annexe with its own kitchen and bathroom can count.
• The figure is the net additional area, so existing floorspace that qualifies for deduction comes off before the comparison.

The exemption is automatic. There is no claim form and no deadline, which is why nobody thinks about it until a council issues a liability notice on a small new dwelling and the applicant cannot see where the charge came from.

Why a new dwelling is charged whatever its size

The policy logic is that a dwelling generates demand for infrastructure regardless of floor area. A two-bedroom bungalow of 72 m² brings the same children to the same schools and the same cars to the same junctions as a larger one, more or less.

So the levy on a small plot scheme is calculated as: net chargeable area, multiplied by the rate for that use in the charging schedule, multiplied by the indexation factor for the year in which permission first permits development. All three come from the council's own charging schedule, which is indexed annually, so read the current version rather than a rate quoted in an old appraisal.

The practical consequence for anyone designing a small unit is that shrinking the plan below 100 m² saves nothing on the levy and costs floor area. Where the plan is being squeezed, it is worth checking the nationally described space standard and the local policy position first, which our note on housing mix and space standards covers.

Net additional floorspace and the deduction that matters

Regulation 40 sets out the calculation. The chargeable area is the gross internal area of the new build, less the gross internal area of any part of an in-use building that is to be demolished or retained and reused as part of the chargeable development.

The definition of an in-use building is where schemes come unstuck. The building must contain a part that has been in 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.

Read that carefully against a typical small site:

1. A barn last used for storage four years ago is not an in-use building. No deduction.
2. An office empty for two years but occupied for eight months in the year before that, within the three-year window, does qualify.
3. A garage in domestic use throughout qualifies, and its area comes off.
4. A building in unlawful use, however long it has been occupied, does not qualify unless the use has become lawful.

Evidence for the six-month lawful use is the applicant's problem, not the council's. Council tax or business rates records, utility bills, tenancy agreements, payroll records and dated photographs are what councils accept. Assemble that at application stage, because proving occupancy two years after the event is hard and the deduction is often worth more than every other saving on the scheme.

Gross internal area is measured to the internal face of the external walls and includes internal walls, plant rooms, stairwells and integral garages. It excludes open balconies, external stores and anything outside the thermal envelope. Disputes about the measurement are common and there is a route for them: request a review of the chargeable amount by the collecting authority, then appeal to the Valuation Office Agency within the period the regulations allow.

Extensions above and below the line

An extension to an existing house of less than 100 m² of new build falls within the minor development exemption, because an extension is not a dwelling. Nothing to claim, nothing to file.

An extension of 100 m² or more is chargeable in principle, but the residential extension exemption exists for exactly that case. It applies where the extension is to a dwelling that is the claimant's principal residence and the claimant holds a material interest in it. The exemption has to be claimed on the relevant form, granted in writing, and the work must not start before the exemption is granted and a commencement notice submitted.

Two traps here. The exemption is not available to a developer extending a house for sale, because the principal residence test fails. And splitting a large extension into two applications to stay under 100 m² does not work, since the council will look at whether the phases form a single chargeable development.

Annexes and the self-build exemption

Two further exemptions sit alongside, and both have hard procedural requirements.

The residential annexe exemption covers a self-contained annexe within the curtilage of the claimant's principal residence. Because an annexe is usually a dwelling for regulation 42 purposes, this exemption is the route rather than the 100 m² threshold. It is withdrawn if the annexe is let out or sold separately within three years of completion.

The self-build exemption covers a new dwelling built by the claimant as their sole or main residence. It is the exemption most small plot builders rely on, and the one most often lost on procedure. The sequence is unforgiving:

1. Assume liability for the levy in writing before commencement.
2. Submit the self-build exemption claim, part one, and wait for the council to grant it in writing.
3. Submit a commencement notice, which the council must receive before any material operation starts. Digging a foundation trench counts.
4. Within six months of completion, submit part two of the claim with the supporting evidence the regulations list, which includes a council tax bill or building regulations completion certificate, a title plan, and either an approved claim under the VAT self-build scheme or a specified insurance or warranty document.
5. Stay in occupation as your main residence for three years from completion, or the exemption is clawed back in full.

Missing the commencement notice is the single commonest way to lose a granted exemption, and there is no discretion to forgive it. Serve it by email with a read receipt and keep the acknowledgement.

Sequence of forms for a small scheme

Where the scheme also triggers planning obligations, the levy sits alongside them rather than replacing them, and the affordable housing position is dealt with separately in the Affordable Housing Statement for schemes that reach the relevant threshold.

The Levelling-up and Regeneration Act 2023 contains powers for an Infrastructure Levy to replace the Community Infrastructure Levy, but those powers are not yet in force nationally, so the 2010 Regulations remain the operative regime. Check the current position on gov.uk before advising a client on a scheme that will take years to build out.

This is general guidance, not legal advice. Levy liability turns on the wording of the council's charging schedule and on the facts of the site, so check the position with your local planning authority or a qualified professional before relying on it.

Working out the supporting documents and the likely levy position at the same time saves a round of revisions. See PlanScope pricing for one free report, £50 per report, or Professional for unlimited reports. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.