The CIL extension exemption and the 100 m² threshold

Most house extensions pay no Community Infrastructure Levy, and the reason is a floorspace threshold rather than any special treatment for homeowners. Under the Community Infrastructure Levy Regulations 2010, development that creates less than 100 m² of new gross internal floorspace and does not create a new dwelling falls within the minor development exemption, and no claim is needed. Cross that line, and the CIL extension exemption becomes something you have to apply for, in writing, and have granted before you start work.

That sequencing is where households lose thousands of pounds. The exemption is generous. The procedure is unforgiving.

When an extension becomes liable

Three situations turn a domestic extension into a liability.

• New gross internal floorspace of 100 m² or more, where the residential extension exemption has not been claimed and granted
• Any development that creates a new dwelling, however small, because the minor development exemption does not apply to new dwellings
• Development in a charging authority area with a rate that applies to the relevant use, since an authority with no adopted charging schedule collects nothing at all

The first step is always to check whether the authority has an adopted charging schedule and what the residential rate is. Rates are set locally, indexed annually, and published by the collecting authority, so look at the current schedule rather than a figure someone quoted two years ago.

A loft conversion within the existing roof usually adds no new floorspace in the sense the regulations use, because the space already exists inside the building envelope. A two storey side extension of 55 m² per floor does add 110 m², and that is the case that needs the claim.

Measuring the 100 square metres

Gross internal area is measured to the internal face of the external walls, and it is the new floorspace that counts.

Two adjustments matter. Existing floorspace in a building that has been in lawful use for a qualifying period can be deducted from the chargeable amount, so demolishing a 20 m² garage to build a 110 m² extension is not the same as building on open garden. And the measurement includes everything inside the envelope: internal walls, circulation, plant cupboards and the awkward space under a stair.

Do the measurement properly at design stage. A scheme at 102 m² and a scheme at 98 m² sit on opposite sides of a procedural cliff, and if you are close to the line it is worth knowing which side you are on before the drawings are issued. Where you are over it, claim the exemption rather than redesigning, since the exemption removes the charge entirely.

The annex route and how it differs

A self contained annex within the curtilage of a home is treated separately. The residential annex exemption applies where the annex is built within the curtilage of the main dwelling, the claimant owns the main dwelling, and they occupy it as their sole or main residence.

The important distinction from the extension exemption is what happens afterwards. An annex exemption can be lost through a disqualifying event in the three years following completion, and an extension exemption has no equivalent clawback. If you are choosing between a linked extension and a detached annex for the same accommodation, that difference belongs in the decision.

Note also that an annex that is in truth a separate dwelling is a different animal again. If the council treats the proposal as creating a new dwelling rather than ancillary accommodation, the minor development exemption falls away and the annex exemption becomes the only route.

Claiming before a spade goes in the ground

The sequence is fixed by the regulations and the collecting authority has little discretion.

1. Submit the assumption of liability form, so the authority knows who is responsible.
2. Submit the exemption claim form for a residential extension or a residential annex, together with the drawings and the floorspace figures.
3. Wait for the authority to notify you in writing that the exemption has been granted.
4. Submit the commencement notice with the intended start date, before any material operation begins.
5. Keep the acknowledgement of each of the above.

The numbered CIL forms are published on the Planning Portal, with separate forms for assumption of liability, the residential extension exemption claim, the residential annex exemption claim and the commencement notice. Form numbering changes from time to time, so download the current versions rather than reusing a saved copy.

Starting work before the exemption is granted is the failure that cannot be fixed. The exemption has to be in place first, and a material operation includes site clearance, demolition and digging foundations, which is earlier than most people think. A missed commencement notice attracts a surcharge and can affect the relief, so serve it even if you believe nothing is payable.

Disqualifying events on an annex

Three years is the clawback period for an annex, running from completion. The events that trigger a withdrawal of the exemption include letting the annex separately, selling it separately from the main dwelling, or using the main dwelling for something other than a single dwelling.

Ordinary family use is fine. A parent living in the annex rent free is fine. A short term let advertised online is exactly the kind of use that produces a demand notice, and the collecting authority does look. Where circumstances change within the three years, tell the authority rather than waiting, because a voluntary disclosure is a better position than a discovered one.

Records worth keeping

Keep a single folder, digital is fine. It should hold the decision notice, the floorspace calculation with the drawing it came from, every CIL form submitted, every acknowledgement received, the commencement notice, and dated photographs of the site on the start date.

That folder is worth having for the conveyancing on a future sale as much as for the levy. Buyers' solicitors ask about CIL, and an unanswered question about whether a liability was ever discharged holds up a sale far longer than the paperwork would have taken.

Larger projects have the same discipline applied at a bigger scale, where the timing of payments matters to cash flow. The mechanics are covered in the note on CIL instalment policies and development cash flow.

If a demand notice arrives anyway

Read it before reacting. A demand notice states the chargeable amount, the payment dates and any surcharges, and it is issued on the basis of the information the authority holds, which may be wrong.

Check four things: whether the floorspace figure matches your measurement, whether existing floorspace has been deducted, whether the exemption claim was received and acknowledged, and whether a surcharge has been added for a procedural failure. Where a surcharge has been imposed, the regulations provide a route to request a review by the collecting authority, and there are appeal routes to the Valuation Office Agency and the Planning Inspectorate on defined grounds within short deadlines. Those deadlines are measured in days rather than months, so act on the day the notice arrives.

Where the levy is correctly charged, the amount is what it is. Levy liability is not negotiable in the way a section 106 contribution can be, and the distinction between the two is explained in the note on section 278 works or a section 106 contribution.

To see which supporting documents your extension needs, and which reports the council will expect with the application, browse the planning report types and start a draft.

This is general guidance, not legal advice, and levy rates, forms and procedures change. Check the current charging schedule and forms with your collecting authority, or take advice from 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.