CIL and residential extensions over the threshold

A household extension of 100 m² or more of new floorspace is liable for the community infrastructure levy in any area with an adopted charging schedule, and there is an exemption that takes it back to nil. The catch is that the exemption has to be claimed and granted before you start, and you have to tell the council in writing before you begin. Miss either step and the exemption is gone, permanently, with a surcharge on top. This is the part of the levy that catches homeowners, and it catches them after the builders are already on site.

The 100 m² threshold, and what it does not cover

The Community Infrastructure Levy Regulations 2010 exempt minor development, meaning development that creates less than 100 m² of new build gross internal floorspace. Most single storey rear extensions and loft conversions fall under it comfortably and no levy arises.

Two things sit outside that comfort. The threshold is measured on gross internal area of new build floorspace, so a two storey side and rear extension on a modest house reaches 100 m² faster than people expect, and a garage conversion contributes nothing because no new build floorspace is created. Measure it properly before you assume you are under.

The other is that the minor development exemption does not apply where the development creates a new dwelling, whatever its size. A self-contained annex of 45 m² with its own kitchen and bathroom is a new dwelling, and the 100 m² threshold does not help it at all.

The levy only bites where the charging authority has adopted a charging schedule. Rates are set per square metre by the authority and indexed annually, so check the current schedule and index figure for your area rather than a rate someone quoted you last year.

The exemption for a householder extension

Regulation 42B provides an exemption for residential extensions. In outline, you qualify where you own a material interest in the dwelling, you occupy it as your sole or main residence, the works extend that dwelling, and they do not create a new dwelling.

That covers the ordinary case: a family enlarging the house they live in. It does not cover a developer extending a house before sale, a landlord extending a let property, or a second home.

The exemption is not automatic and it is not granted with the planning permission. It is a separate claim, made to the collecting authority on the prescribed form, and it has to be granted in writing before development begins.

Annexes are a different exemption

Regulation 42A deals with residential annexes, and it exists precisely because an annex is a new dwelling and therefore outside the minor development exemption.

The conditions are similar in shape. The annex is within the curtilage of the main dwelling, it is ancillary to it, you own a material interest in the main dwelling and you occupy it as your sole or main residence.

Then there is a tail. For a defined period of three years after the annex is completed, certain events disqualify the exemption and make the levy payable after all. Letting the annex out, or selling it separately from the main house, are the obvious ones. If the plan is to build an annex for a parent now and let it in two years, price the levy in rather than claiming the exemption and hoping.

Claim it, then tell them you are starting

The sequence matters more than the paperwork, and it runs like this.

1. Submit an assumption of liability form, naming who will pay if the levy becomes payable
2. Submit the exemption claim on the correct form, and wait for the authority to grant it in writing
3. Serve the notice required by regulation 67, on Form 6, before you start work, giving the intended start date
4. Only then begin development

Step three is where householder exemptions are lost. That notice is a short form and it is mandatory. Where no valid notice is served, the authority fixes the start date itself, the exemption is lost, any instalment policy no longer applies and the whole amount becomes due with a surcharge.

There is no discretion to restore an exemption after work has started. That is the harsh part of this regime and it has been tested repeatedly. Diarise it, serve it by email with a delivery receipt, and keep the acknowledgement with the building contract.

Permitted development can still be chargeable

An extension built under permitted development rights, with no planning application at all, can still be liable. Where development is carried out under a general consent and creates 100 m² or more of new build floorspace, a notice of chargeable development has to be submitted to the authority before work starts.

Homeowners almost never know this, and it usually surfaces later, when a solicitor's search on a sale reveals an unpaid liability with accrued surcharges. A large single storey rear and side wrap-around extension, built lawfully under permitted development, is exactly the case where it happens. If you are building 100 m² or more without a planning application, check whether your authority charges the levy and whether a notice is required. Sorting out an unpaid liability years later sits alongside the wider cost of retrospective development in the list of expenses nobody budgets for.

Netting off existing floorspace

Where you demolish part of the house, or convert an existing building, the floorspace of a qualifying existing building can be deducted from the chargeable amount.

The building has to have been in lawful use for a defined minimum period within the three years ending on the day planning permission first permits the chargeable development. A derelict outbuilding that has not been used for a decade will not qualify, which is counter-intuitive to most homeowners, who assume that replacing a building is neutral.

So keep evidence of use. Council tax records, utility bills, insurance, photographs and tenancy agreements all help, and they need to relate to the relevant window rather than to the building's history generally. Set the calculation out on the application drawings, with existing and proposed gross internal areas tabulated floor by floor, and put the same figures in the Planning Statement so the levy officer and the case officer are working from one set of numbers.

Surcharges, reviews and appeals

The regulations provide surcharges for failing to assume liability, failing to serve the start notice, late payment and disqualifying events. They are set out in the regulations and are applied as a matter of course rather than as a discretionary penalty.

Three routes exist if you disagree. A request to the collecting authority to review the calculation of the chargeable amount, which has a short deadline running from the demand notice. An appeal to the Valuation Office Agency on the chargeable amount itself. And appeals to the Planning Inspectorate on surcharges, on a start date fixed by the authority, and on the refusal or withdrawal of an exemption.

All of them are tight on time. Read the demand notice on the day it arrives and note the deadlines, because a valid ground of appeal that is raised late is worth nothing.

Finally, budget the levy as a cash item due when work starts, alongside the other things that land at the same moment, including the utility connection costs in a development budget. It is not spread across the build unless the authority operates an instalment policy and you met your notice obligations. And if a dispute does escalate, the way appeal costs are assessed and paid is worth understanding before you commit to a fight.

This is general guidance, not legal advice. Levy rates, indexation and surcharge amounts change, and the exemption conditions are detailed, so check the position with your collecting authority or a qualified professional before relying on it. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.

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