CIL exemption for extensions and residential annexes

An extension to your own home of less than 100 m2 gross internal area, which does not create a separate dwelling, falls outside the Community Infrastructure Levy altogether. At 100 m2 or more it becomes chargeable, and you have to claim the residential extension exemption in writing before you start work. A self-contained annexe is chargeable at any size, because it creates a dwelling, and it needs its own exemption claim.

The CIL exemption for extensions and annexes is generous. It is also the exemption most often lost to a missed form, and once it is lost there is no discretion to reinstate it.

The floorspace threshold for extensions

Gross internal area is measured to the inside face of the external walls, and it includes everything inside that line. Internal partitions, staircases, plant, integral garages and circulation all count. Designers who work in net internal area out of habit routinely under-measure by several square metres, which is enough to move a scheme across the threshold without anyone noticing.

Below 100 m2, and provided no new dwelling is created, the works are simply not chargeable development. There is nothing to claim and no form to file.

At or above 100 m2, the works are chargeable, and the residential extension exemption exists to take them back out of charge. The conditions are specific:

• The extension must be to an existing dwelling.
• It must not create a new dwelling, which means no separate self-contained accommodation.
• The person claiming must own a material interest in the dwelling and occupy it as their sole or main residence.
• The claim must be made and granted before development starts.

Two of those conditions catch people. A landlord extending a let house cannot claim, because they do not occupy it. And an extension that includes a self-contained ground-floor flat for a relative is a new dwelling, so the extension exemption does not apply and the annexe exemption has to be considered instead.

When an extension becomes chargeable anyway

The threshold is applied to the chargeable development as a whole, so a scheme cannot be split to stay below it. Where an extension and a detached outbuilding are permitted together, the areas are counted together if both form part of the same chargeable development.

Conversion of an existing garage or loft does not usually add gross internal area, because the floorspace already exists inside the building envelope. Demolishing and rebuilding is different, and there the in-use test for deducting existing floorspace applies, requiring six continuous months of lawful use in the three years before permission.

Where permitted development rights are being used rather than a planning application, the levy can still apply. General consent development creating 100 m2 or more, or a new dwelling, is chargeable, and the collecting authority expects a notice of chargeable development before work starts. This catches larger permitted development schemes, including some conversions.

The separate annexe exemption

A residential annexe creates a dwelling, so the 100 m2 threshold does nothing for it. There is a specific exemption instead, and it applies where the annexe is built within the curtilage of the main dwelling, is a single dwelling, and the claimant owns a material interest in the main dwelling and occupies it as their sole or main residence.

The annexe does not have to be attached. A garden annexe qualifies, provided it sits inside the curtilage of the house rather than on a separate plot with its own access and boundaries, which is the distinction councils look at hardest.

If you are planning to sever the garden later, or to sell the annexe separately, stop and take advice first. The exemption is built on the annexe remaining part of the same household holding, and the clawback provisions are written accordingly.

Claiming before development starts

The sequence is unforgiving and it is worth writing on the wall of the site hut:

1. Assume liability using the assumption of liability form.
2. Submit the exemption claim on the correct form for an extension or for a residential annexe.
3. Wait for the collecting authority to grant the exemption in writing.
4. Submit the commencement notice, naming the intended start date, and get an acknowledgement.
5. Start work only after all of that is in place.

Step four is where exemptions die. If development starts before a valid commencement notice is received by the collecting authority, the exemption is withdrawn and the full chargeable amount becomes payable, usually with a surcharge on top. Digging a trench for foundations counts as starting. So can demolition where it forms part of the development.

There is no appeal against the loss of an exemption for a missed commencement notice on the merits. The only routes are the formal appeal grounds in the regulations, which are narrow.

Records to keep after completion

Keep the exemption decision letter, the acknowledged commencement notice and the building control completion certificate together, in one place, for at least three years after completion. You will need them if the property is sold, if the council queries the position, or if a disqualifying event has to be reported.

Councils increasingly check. A completion certificate that shows a self-contained unit where the exemption was granted for an extension is exactly the sort of mismatch that prompts a letter.

Illustrative example. A homeowner builds a 110 m2 wraparound extension with a bedroom, shower room and small kitchenette at the rear, intended for an elderly parent. Because the kitchenette and separate entrance make the rear part capable of independent occupation, the collecting authority treats it as a new dwelling, and the extension exemption granted earlier no longer fits what was built. Whether a space is self-contained turns on its physical arrangement rather than on how the family uses it, so the point is worth settling at design stage.

Where the annexe is later let or sold

The annexe exemption carries a clawback period of three years from completion. If a disqualifying event happens inside that period, the exemption is withdrawn and the levy becomes payable.

The events that count are essentially about the annexe ceasing to be ancillary to the main house. Letting the annexe out, or selling the annexe separately from the main dwelling, or selling the main dwelling separately from the annexe, all fall into that category. Selling the house and annexe together to the same buyer does not.

There is a duty to notify the collecting authority of a disqualifying event, within a short period set by the regulations, and failing to notify attracts a surcharge of its own. Set a reminder in the third year rather than relying on memory.

Nobody enjoys discovering this at the point of sale. A conveyancer will ask, and an unpaid levy secured as a land charge will hold the transaction up until it is resolved.

For a straightforward extension, the levy usually turns out to be a form-filling exercise rather than a cost. Budget your attention accordingly, and spend the saved effort on the documents that decide whether permission is granted at all. The full list of report types shows what a householder scheme in a constrained location can attract, and PlanScope pricing sets out what drafting one costs. If you want a site-specific view before you commission anything, enter your postcode and work through the guided questions.

This is general guidance, not legal advice. Exemption forms, deadlines and surcharge amounts change, so check the position with your collecting authority or a qualified professional before relying on it.

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