Do you pay CIL on a house extension?

The 100 square metre rule

(1) Liability to CIL does not arise in respect of a development if, on completion of that development, the gross internal area of new build on the relevant land will be less than 100 square metres. (2) But paragraph (1) does not apply where the development will comprise one or more dwellings. (3) In paragraph (1) "new build" means that part of the development which will comprise new buildings and enlargements to existing buildings.

The threshold is the total new build in the development, on every floor, including any new outbuilding in the same permission. Gross internal area is not defined in the Regulations. Government guidance says charging authorities decide what to include, and that the Valuation Office Agency normally uses the definition in the RICS Code of Measuring Practice (planning practice guidance, paragraph 100). Brighton and Hove City Council, for example, describes it as measured to the internal face of the perimeter walls on each floor.

The residential extension exemption

A person (P) is exempt from liability to pay CIL in respect of development if (a) P owns a material interest in a dwelling ("main dwelling"); (b) P occupies the main dwelling as P's sole or main residence; and (c) the development is a residential annex or a residential extension. ... (3) The development is a residential extension if it (a) is an enlargement to the main dwelling; and (b) does not comprise a new dwelling.

A material interest is a freehold, or a lease whose term expires more than seven years after the day planning permission first permits the development (regulation 4(2)). A landlord extending a house they let does not live in it, so cannot use this exemption.

The claim must (a) subject to paragraph (3A), be received by the collecting authority before commencement of the chargeable development ... (3) Subject to paragraph (3A), a claim under this regulation will lapse where the chargeable development to which it relates is commenced before the collecting authority has notified the claimant of its decision on the claim.

Two conditions follow. The claim must reach the council before work starts, and work must not start until the council has told you in writing that the exemption is granted. Form 9 itself makes you tick a box confirming you understand the claim lapses if development starts before the decision.

What counts as starting work

Development commences on "the earliest date on which any material operation begins to be carried out on the relevant land" (regulation 7(2)). Regulation 7(6) takes the meaning of material operation from section 56(4) of the Town and Country Planning Act 1990. It includes:

• any work of construction in the course of the erection of a building;
• any work of demolition of a building;
• digging a trench to contain foundations;
• laying an underground main or pipe to the foundations or to such a trench.

Demolishing an old conservatory the extension replaces, or digging the first foundation trench, can therefore start the development for CIL purposes. So can work in breach of the permission that is later regularised by retrospective permission: under regulation 7(5)(a) that development is treated as commencing on the day the retrospective permission is granted, and government guidance says no exemption, other than the minor development exemption, can then be obtained (paragraph 047a).

The claim, in order

1. Check whether your council charges CIL. If it does, its website should have a charging schedule and an annual CIL rate summary. In London the Mayor's CIL applies as well, and the same exemptions cover it.
2. Measure the new gross internal area. Under 100 m2 with no new home: stop here, nothing to claim.
3. If it is 100 m2 or more, send Form 9, the residential extension exemption claim, to the council's CIL team (the collecting authority).
4. Wait for the written decision. Do not start any material operation before it arrives.
5. Once the exemption is granted, no commencement notice is needed (regulation 67(1A)(aa)). Keep the decision letter with the planning permission.

If the extension is permitted development, not a planning application, the same thresholds apply. Chargeable development under permitted development needs a notice of chargeable development before it starts (regulation 64(2)), but not where regulation 42 applies or an extension exemption has been granted (regulation 64(1A)).

What this means for you

For most extensions the answer is no CIL and no paperwork. For a larger one, the exemption is available to any owner-occupier, and the only way to lose it is timing: claim with Form 9 and wait for the decision before anything happens on site. The CIL checker runs through these questions in order.

Parallel prepares planning applications for extensions. The exemption claim is made in the owner's name, because regulation 42A turns on who owns and lives in the house.