CIL relief for a residential extension
The minor development exemption removes CIL from extensions below 100 square metres. Larger extensions and annexes need relief claimed and granted before any work begins.
Most house extensions pay no Community Infrastructure Levy at all, because the levy does not bite on new build floorspace of less than 100 square metres gross internal area unless a dwelling is created. That is the minor development exemption in regulation 42 of the Community Infrastructure Levy Regulations 2010, and it applies automatically with no form to complete. Above 100 square metres, an extension to your own home can still be free of charge, but only through a relief that has to be claimed before work starts and granted by the authority in writing.
The difference between an automatic exemption and a claimed relief is where householders lose money. Miss the claim, start the work, and a five figure demand arrives for something that would have cost nothing.
When an extension becomes levy liable at all
Two things have to line up before levy is payable on a householder project. The authority must have an adopted charging schedule covering the area, and the development must exceed the minor development threshold or create a dwelling.
Many charging schedules set a nil rate for extensions to existing dwellings, so the arithmetic never arises. Check the schedule on the authority's website before worrying about the exemptions, and check the zone, because rates often vary across a district.
Where a rate does apply, the measure is the gross internal area of new build floorspace in the chargeable development. For a straightforward extension, that is the extension itself, measured to the internal face of the external walls, including any attached garage and any mezzanine or additional floor created inside the existing envelope.
The minor development threshold and how it is measured
Regulation 42 removes liability where the gross internal area of new build on the chargeable development is less than 100 square metres. One condition sits on it: the exemption does not apply where the development comprises one or more dwellings.
So a 95 square metre extension to a house is outside the levy. A 95 square metre building that is a new dwelling is not, and neither is a conversion that creates a flat, however small.
Measurement matters more than people assume, because the threshold is a cliff rather than a slope. An extension of 101 square metres is liable on the whole of its floorspace, not on the one square metre above the line. Have the areas measured properly and shown on the drawings before submission, since an area schedule on the application drawings is what the authority will use.
Floorspace in an existing building that has been in lawful use can be deducted from the chargeable amount. The test is demanding, requiring continuous lawful use for a period the regulations specify within the years before permission first permits development, and authorities apply it strictly. Keep utility bills, council tax records and photographs for any building you intend to claim against. The position where an extension does cross the threshold is covered in CIL and residential extensions over the threshold.
Relief for residential extensions and the claim process
Regulation 42A provides an exemption for a residential extension to a dwelling. It is available where the person claiming owns a material interest in the dwelling and occupies it as their sole or main residence, and it covers an extension that does not create a new dwelling.
The procedure is unforgiving and has three steps that must happen in order.
- Submit an assumption of liability form to the collecting authority.
- Submit the exemption claim on the national form, before any work starts, and wait for the authority to notify that the claim is granted.
- Submit a commencement notice before work starts on site.
Starting work before the authority has granted the claim invalidates it. That is the single most common failure, usually because a builder turns up early or because groundworks are treated as preparation rather than a material operation. A trench dug for foundations is a material operation.
Do not rely on the planning application as notice of anything. The levy process is administered separately, often by a different team, and the planning case officer will not chase you.
Annexes and the separate relief that applies
A self contained annexe within the curtilage of a dwelling has its own exemption, also claimed before work starts and granted by the authority. It covers an annexe built within the grounds of the main dwelling and occupied in connection with it.
The annexe relief carries a clawback that the extension relief does not. A disqualifying event within three years of completion removes it, and the events include letting the annexe separately and selling it separately from the main dwelling. The full chargeable amount then becomes payable.
That is worth understanding before building a granny annexe as an investment. An annexe let on an assured shorthold tenancy a year after completion triggers the clawback, and the authority finds out through council tax banding.
If the annexe is intended to be a separate dwelling from the outset, it is a dwelling for levy purposes and the minor development exemption is unavailable regardless of its size.
Why a commencement notice still matters where relief applies
The commencement notice tells the authority the date development starts. It is required even where no money is payable, and it is required even where an exemption has been granted.
The treatment of a missed commencement notice has been amended since the regulations were made, and in many cases the consequence is now a surcharge rather than outright loss of the exemption. The position is specific and has changed more than once, so check the current consolidated regulations rather than relying on summaries written a few years ago.
Either way, submitting it costs nothing and takes ten minutes. Send it by email with a read receipt and keep the acknowledgement, because disputes about whether a notice was received are common and the burden falls on you.
What happens if the annexe is later sold separately
Selling an annexe separately within the clawback period makes the full levy payable, calculated on the original figures, and the authority issues a demand notice to the person who claimed the relief.
Plan around it in one of three ways. Keep the annexe in the same title and the same occupation for the clawback period. Price the potential levy into the sale if an earlier disposal is likely. Or build the annexe as a dwelling from the start, pay the levy and have no restriction on what you do with it afterwards.
There is also a practical point about mortgages and sales. A solicitor acting for a buyer will ask about levy liability and exemptions, so keep the paperwork in a form you can hand over. A missing exemption notice can stall a sale for weeks while the authority confirms the position.
Paperwork to keep for the file
Keep six documents together, scanned, in one folder.
The planning permission with the approved drawings and the area schedule. The liability notice from the authority. The assumption of liability form and its acknowledgement. The exemption claim and the authority's written grant. The commencement notice and its acknowledgement. And evidence of occupation as a sole or main residence covering the relevant period, such as council tax bills.
That folder answers every question a conveyancer, a lender or a future authority will ask. It also gives you the evidence to resolve the charge if a liability notice is issued in error, which happens more often than you might expect where a scheme is varied under section 73 and a new liability notice is generated automatically.
See PlanScope pricing if you are budgeting the supporting documents for an extension alongside the application fee and the building control charges, which are set out in building control charges alongside the planning fee.
This is general guidance, not legal advice. Check the position with your local planning authority or a qualified professional before relying on it.
Enter your postcode on PlanScope to see which documents an extension application is likely to need.
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Frequently asked questions
Do I pay CIL on a house extension?
Usually not. Regulation 42 of the Community Infrastructure Levy Regulations 2010 removes liability where the gross internal area of new build is less than 100 square metres and no dwelling is created. The exemption is automatic with no form to complete. Many charging schedules also set a nil rate for extensions.
How is the 100 square metre threshold measured?
As gross internal area of new build floorspace in the chargeable development, measured to the internal face of external walls. It includes an attached garage and any mezzanine or additional floor created inside the existing envelope. The threshold is a cliff, so 101 square metres is liable on the whole floorspace.
When must I claim relief for a larger extension?
Before any work starts, and the authority must grant the claim in writing before you begin. Submit the assumption of liability form first, then the exemption claim on the national form, then a commencement notice. Starting before the grant invalidates the claim, and digging foundations counts as starting.
Is an annexe treated the same as an extension?
It has its own exemption, also claimed before work starts, but it carries a three year clawback that the extension relief does not. Letting the annexe separately or selling it separately from the main dwelling within three years of completion removes the exemption and makes the full amount payable.
Do I need a commencement notice if no money is payable?
Yes. The commencement notice is required even where an exemption has been granted and nothing is due. The consequence of missing it has been amended since the regulations were made, and in many cases is now a surcharge. Check the current consolidated regulations and keep the acknowledgement.
Sources and further reading
This article is general guidance about the planning system in England and, where stated, the rest of the UK. It is not legal or professional advice. Check your local planning authority’s validation requirements and take professional advice for your specific site.