Where CIL applies, a new dwelling can be chargeable even below 100m², although a qualifying residential-annexe exemption may remove the charge if the correct process is completed before work starts. [1] [2] [3]
First decide what the annexe will be
An extra room attached to the house, a detached annexe and an independent garden dwelling are not interchangeable planning proposals. The ordinary incidental-outbuilding permission does not authorise a new residential annexe. Describe who will occupy it and how it relates to the main household before choosing an application route. [1]
Where a council permits only ancillary accommodation, a condition may prevent it becoming a separate residential unit. Check the actual wording rather than assuming later letting or separate sale will be possible. The word “granny” does not itself establish an exemption. [4]
Do not use the garden-room building exemption
Sleeping accommodation falls outside the small-detached-building exemptions. The design therefore needs a building-regulations assessment rather than reliance on a supplier's standard shed specification. Establish the approval route before construction and keep the eventual completion documentation. [5] [2]
For a proposal that creates a new dwelling, the Future Homes Standard changes take effect on 24 March 2027, subject to transitional arrangements. An annexe is not automatically a new dwelling for these purposes: its classification and scope of work need to be established. The new-home standard is not a blanket requirement to rebuild an existing garden room. [6]
CIL: eligibility is not the same as having an exemption
The residential-annexe exemption has specific ownership and occupation conditions. The claimant must have the required interest in the main dwelling, occupy it as their sole or main residence, and meet the rules for an annexe wholly within its curtilage comprising one new dwelling. The annexe occupant need not be a relative. [3]
Do not assume that a small floor area or family occupation automatically results in a nil bill. Ask the collecting authority to assess the project and confirm the appropriate exemption route. [3]
Complete the paperwork before starting
- Provide the CIL additional information and arrange assumption of liability.
- Submit Form 8, the residential-annexe exemption claim, with the required evidence.
- Obtain the exemption decision before commencing the development.
- Submit Form 6, the commencement notice, before starting on site. A late or missing notice can attract a surcharge. [7] [3]
During the three years following completion, letting the annexe, selling either building separately, or changing the main house from a single dwelling can trigger clawback. Retain the exemption and completion records when discussing any later occupation or ownership change. [7]
Keep biodiversity net gain separate
From 6 August 2026, the BNG exemption for sites of 0.2 hectares or less uses the area within the application's red-line boundary, not the annexe's floor area. It is unavailable where the development impacts priority habitat. The self-build and custom-build BNG exemption was removed for new applications made on or after that date. Householder applications remain a separate exemption category. None of these BNG rules automatically grants CIL relief. [8]