Costs & Materials
CIL on London house extensions: when you pay, and how the residential exemption works
A large house extension in London can attract Community Infrastructure Levy on top of build costs. Most owner-occupiers can claim a residential exemption, but only if the paperwork is done before work starts. Here is how the rules work in 2026.
The Community Infrastructure Levy (CIL) is a charge on new floorspace that helps fund local infrastructure such as schools, parks and transport. In London you may face both a borough CIL and the Mayor of London's CIL (MCIL2). For homeowners planning a substantial extension or a residential annex, the levy is easy to overlook until a liability notice arrives. This guide summarises the national rules from the government's CIL planning practice guidance, how they interact with London's two-tier charge, and the residential extension and annex exemptions. It is general information, not advice on your particular project.
When is a house extension potentially liable?
In areas that have adopted a charging schedule, CIL may be payable on development that creates new or additional internal floorspace where the gross internal area of new build is 100 square metres or more. A new dwelling of any size can also be liable. Where the calculated liability is less than £50, it is treated as zero. Most extensions under 100 square metres that do not create a new dwelling fall under the minor development exemption and do not pay CIL — but once you cross 100 square metres, or create a new dwelling such as an annex, you need to look carefully at liability and relief.
CIL is linked to planning permission as defined in the CIL Regulations. It can also apply to permitted development: if you rely on permitted development for a chargeable scheme, you may need to submit a Notice of Chargeable Development (Form 5) before starting. A Lawful Development Certificate alone does not trigger or cancel CIL. See our guides to permitted development for house extensions and larger home extension prior approval.
London's two charges: borough CIL and Mayoral CIL
London is unique in having a strategic Mayoral CIL as well as borough (or Mayoral development corporation) charges. Boroughs collect Mayoral CIL on the Mayor's behalf. Rates are set in each charging schedule and are indexed each calendar year using the RICS CIL Index. For illustration only, the London Borough of Haringey's Annual CIL Rate Summary for 2026 shows indexed borough residential rates of £443.52, £276.16 and £60.24 per square metre in its Western, Central and Eastern zones respectively, and an indexed Mayoral CIL (MCIL2) rate of £72.73 per square metre for Band 2 (the band that includes Haringey), using the 2026 index figure of 400 against MCIL2's base index of 330. Other borough bands and schedules differ, so always check your own council's current annual CIL rate summary and the Mayor of London's CIL pages. Treat any pounds-per-square-metre figure as indicative until your collecting authority confirms the chargeable amount for your permission.
In practice that means a large extension in an inner borough such as Islington, Camden or Hammersmith & Fulham, or an outer borough such as Ealing or Croydon, can attract different combined rates. The collecting authority for most householder applications is your London borough.
The residential extension exemption
People who enlarge their own home can be exempt from CIL if they meet the criteria in regulations 42A and 42B of the CIL Regulations 2010 (as amended). In summary:
- you must own a material interest in the main dwelling (freehold, or a lease with more than seven years remaining when permission first permits development);
- you must occupy that dwelling as your sole or main residence; and
- the development must be a residential extension: an enlargement of the main dwelling that does not comprise an additional dwelling.
You claim on CIL Form 9: Residential Extension Exemption Claim, available from the Planning Portal CIL forms page. The collecting authority must grant the exemption before development commences. If you start work first, you are liable for the full levy charge even if you would otherwise have qualified. For a residential extension exemption there is no requirement to submit a commencement notice (regulation 67(1A)), but you still must not start until the exemption has been granted in writing.
Residential annexes are different
A residential annex — one new dwelling wholly within the curtilage of your main home — has its own exemption, claimed on Form 8. You still need the exemption granted before commencement. Unlike a simple extension, an annex exemption also requires a Commencement Notice (Form 6) before work starts; missing it attracts a surcharge (generally 20% of the notional chargeable amount, capped at £2,500) rather than automatic loss of the exemption where the liability notice was issued on or after 1 September 2019. Letting the annex, or selling it separately from the main house, within three years of the compliance certificate can claw the exemption back.
Why timing still matters in 2026
On 25 June 2026 the Minister of State for Housing and Planning, Matthew Pennycook, wrote to CIL charging authorities about householders who had faced unexpected CIL bills after procedural mistakes — for example starting work before an exemption was granted, or needing retrospective permission under section 73A, which cannot usually obtain CIL relief. The letter, published on GOV.UK, said the government would consult on targeted changes to make the regime clearer for householders and self-builders, and reminded councils of their discretion over enforcement and, in exceptional hardship cases, ex gratia payments from general funds. Those reforms are not yet in force. Until amended regulations take effect, the existing rule remains: claim and obtain the exemption before you dig.
Practical checklist for London homeowners
- Check whether your borough (and the Mayor) charge CIL on residential floorspace, and read the current annual rate summary.
- Ask your designer for the proposed gross internal area of new build. If it is under 100 square metres and you are not creating a new dwelling, you may rely on the minor development exemption — still confirm with the council if you are unsure.
- If you need the residential extension or annex exemption, submit Form 9 or Form 8 early, with the planning application or as soon as permission (or a chargeable permitted-development scheme) is in place, and wait for written confirmation.
- Do not treat planning permission, prior approval or a Lawful Development Certificate as a substitute for the CIL forms.
- Budget separately for build costs. For indicative 2026 London construction ranges, see our house extension cost guide.
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Sources: GOV.UK: Community Infrastructure Levy planning practice guidance (including paragraphs on liability, minor development, and residential annex/extension exemptions); CIL Regulations 2010, regulation 42A; Planning Portal CIL forms (Forms 5, 6, 8 and 9); London Borough of Haringey: CIL Charging Schedule and Annual CIL Rate Summary for 2026 (illustrative indexed borough and MCIL2 Band 2 rates); Mayor of London: Mayoral Community Infrastructure Levy; MHCLG letter on CIL charges on householders, 25 June 2026. This guide is a summary, not legal advice; confirm liability and forms with your collecting authority.
Frequently asked questions
Do I pay CIL on a house extension under 100 square metres?
Usually not, if the extension does not create a new dwelling: development under 100 square metres of new build generally benefits from the minor development exemption. Confirm with your borough if your scheme is close to the threshold or includes an annex.
How do I claim the residential extension CIL exemption in London?
Submit CIL Form 9 to your borough (the collecting authority) and obtain written confirmation that the exemption is granted before any work starts. Starting first makes you liable for the full charge.
Does Mayoral CIL apply to householder extensions?
Mayoral CIL (MCIL2) applies across London to chargeable development alongside any borough CIL, unless an exemption or relief applies. Claim the residential extension or annex exemption before commencement if you qualify.
Have the June 2026 CIL reforms changed the rules yet?
No. The government's 25 June 2026 letter proposed a consultation on clearer rules for householders, but until amending regulations are made and in force you must still obtain any exemption before starting work.
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