Extensions below 100 m² which do not form part of development creating a dwelling normally have the minor-development exemption. Larger qualifying extensions and new annex dwellings require the correct claim and written approval before work starts. [1][2][3]
Identify which exemption fits
For the household exemptions, the applicant must occupy the main house as their principal residence and own a qualifying interest in it. The CIL definitions distinguish an extension, which enlarges that home without creating another dwelling, from an annex, which is one new dwelling within its curtilage. Calling a separate dwelling a “garden room” does not change which facts must be assessed. [1]
For an extension below 100 m², check that the proposal really falls within the minor-development exemption. For a larger extension, check the household extension exemption instead. An annex creating a new dwelling does not escape CIL simply by being smaller than 100 m². [1]
Planning permission and the CIL assessment answer different questions. Do not treat an acceptable layout or the word “ancillary” on a drawing as written confirmation that a CIL exemption has been granted.
Apply before authorising the start
Use Form 9 for a residential extension exemption and Form 8 for a residential annex exemption. Both forms warn that the exemption must be obtained before development commences. Sending the claim is not enough: wait for the collecting authority's written confirmation. [3][2]
For a practical pre-start check, keep the permission, CIL reference, submitted claim and exemption decision together. Confirm that they describe the same scheme. Give the builder a clear instruction not to begin until the required CIL steps, as well as the project's other pre-start requirements, have been completed.
The commencement-notice difference
For a residential extension exemption where the liability notice, or relevant revised notice, was issued on or after 1 September 2019, Form 9 states that a CIL commencement notice is not required. The form distinguishes older notices, for which different requirements apply. [3]
An exempt annex still needs Form 6 received before commencement. For notices on or after 1 September 2019, failing to give that notice can produce a surcharge even though an exemption was granted. Earlier notices can have more severe consequences, including loss of relief. Check the date of the actual notice rather than assuming every historic permission follows today's procedure. [2][4]
Annex exemptions can be withdrawn
An annex has a three-year clawback period, starting with the date of its compliance certificate. Letting the annex or selling it separately from the main home during that period can withdraw the exemption. Selling the main home separately can also trigger clawback, as can the main residence ceasing to be a single dwelling. The council must be notified of a disqualifying event within 14 days. [1]
These annex rules should not be copied onto the extension exemption as though the two categories were identical. Before a sale, letting or subdivision, have the annex's exemption conditions checked against the proposed transaction.
Proposed procedural reforms
MHCLG's 25 June 2026 letter announces proposed changes intended to address householders' procedural errors. It does not itself grant retrospective exemption or remove the need to obtain approval before starting. Check the operative exemption procedure and commencement of any replacement rules before authorising the work. [5]