Community Infrastructure Levy: How It's Calculated and the Traps That Catch Developers Out

Of all the charges that land on a development site, the Community Infrastructure Levy is the one most likely to be right on the numbers and wrong on the process. CIL is not complicated to calculate once you know the rate, but the reliefs that make it manageable — self-build, social housing, the existing floorspace deduction — all depend on paperwork filed before a spade goes in the ground. Miss the sequence, and a scheme that should have owed nothing can end up paying the full charge plus a surcharge.

What CIL is, and who charges it

The Community Infrastructure Levy is a charge that local planning authorities can choose to apply to new development, to help fund infrastructure such as roads, schools and green space needed to support growth in the area. It is not universal — a local authority has to have formally adopted a CIL charging schedule before it can levy it, and not every authority has done so, some relying instead on Section 106 agreements alone. The first step on any scheme is confirming whether the relevant charging authority actually operates CIL, and at what rate for the proposed use.

How the charge is calculated

CIL is charged per square metre of net additional gross internal floorspace created by a development, at rates set out in the charging authority's published charging schedule. Rates are not uniform — they are set separately for different uses (private housing is typically charged at a materially higher rate than employment or retail space) and often vary by zone within the same authority. The rates are index-linked and updated annually by reference to a national build cost index, so the figure that applied when a scheme was first appraised may not be the figure due by the time it reaches building control.

The word "net" matters. Where a development involves demolishing an existing building, the floorspace of that building can usually be deducted from the new floorspace when calculating the chargeable amount, provided it has been in continuous lawful use for at least six months within the previous three years. On a conversion or redevelopment site with a long-vacant existing building, that six-month test can be the difference between a substantial CIL bill and a much smaller one, which makes the history of a site's occupation worth checking early, not after the charging authority has already issued its liability notice.

The reliefs and exemptions worth checking

Several categories of development can reduce or remove a CIL liability entirely:

• Minor development. A development of a single dwelling under 100 square metres that does not create a new home (for example, a modest extension) is exempt outright.
• Residential annexes and extensions. Extensions to an existing home, and self-contained annexes within the grounds of a main dwelling, can qualify for relief.
• Self-build housing relief. A genuine self-build home, or a self-build communal project, can be fully relieved — but the relief carries a clawback if the property is sold or let within a set period, so it is not a route to relief on a development intended for disposal.
• Charitable relief. Development by a charity for its charitable purposes can qualify, subject to state aid-style restrictions where commercial use is also involved.
• Social housing relief. Qualifying social and affordable housing, broadly mirroring the categories used elsewhere in property tax, can be relieved in full or in part.

None of these apply automatically. Every one of them has to be formally claimed with the charging authority, and in most cases the relief needs to be granted before development commences — a scheme that starts work first and applies for relief afterwards has usually already lost it.

The paperwork trap that voids them all

This is where CIL causes the most damage, and it has nothing to do with whether a scheme was ever entitled to relief in the first place. Before any work starts on site, the party liable for CIL must submit an Assumption of Liability notice, and then a separate Commencement Notice confirming the actual start date, to the charging authority.

Miss that sequence, and the consequences stack up quickly: any self-build, charitable or social housing relief already granted is automatically disqualified, the right to pay the levy in instalments (where the authority offers one) is lost, the full amount becomes payable immediately rather than on the agreed schedule, and a surcharge equal to 20% of the chargeable amount or £2,500, whichever is lower, is added on top. It is entirely possible for a scheme that would have owed nothing under a correctly claimed relief to end up paying the levy in full, plus a penalty, purely because the notice went in late or not at all. On a site where multiple contractors or phases are involved, confirming who is actually filing the Commencement Notice, and when, is worth a five-minute conversation before groundworks begin.

CIL and Section 106 together

CIL and Section 106 obligations commonly apply to the same scheme, but they are meant to serve different purposes rather than duplicate each other. CIL receipts are pooled by the charging authority against its published infrastructure list — the general things a growing area needs. A Section 106 agreement, by contrast, is negotiated site by site to mitigate the specific impact of that development, and is still the main route for securing on-site affordable housing. The same piece of infrastructure should not be funded through both, and a charging authority's Regulation 123 (or equivalent) infrastructure list is the reference point for checking that a Section 106 ask is not duplicating something CIL is already meant to cover.

What this means for appraisal

CIL behaves like a fixed cost rather than a tax on profit, in the same way the Building Safety Levy does, which means it needs to sit in a scheme's viability appraisal from the outset rather than as a late addition once planning permission is granted. For any scheme where reliefs might apply — a self-build element, an affordable housing component, or existing floorspace on the site — building the relief claim and the notice sequence into the project programme, alongside whoever is managing the building control process, avoids the single most common way developers end up paying CIL they never actually owed.

Common questions

How is the Community Infrastructure Levy calculated?

CIL is charged per square metre of net additional gross internal floorspace created by a development, at rates set by the local charging authority in its CIL charging schedule. Rates vary significantly by area and by use (for example, private housing is typically charged at a higher rate than employment space), and are index-linked each year. Existing floorspace that has been in continuous lawful use for at least six months in the preceding three years can usually be deducted from the chargeable amount, even where it is being demolished.

What CIL exemptions and reliefs are available?

The main reliefs are self-build housing relief, relief for residential annexes and extensions, charitable relief, and social housing relief. Development of a single dwelling under 100 square metres that does not create a new home is also exempt as minor development. All of these must be formally claimed and, in most cases, granted before development begins; none apply automatically just because the development would otherwise qualify.

What happens if I don't submit a CIL Commencement Notice?

Starting work without first submitting an Assumption of Liability notice and a Commencement Notice to the charging authority is treated as unauthorised commencement. It voids the right to pay by instalments, disqualifies any self-build, charitable or social housing relief that had been granted, and triggers a surcharge equal to 20% of the chargeable amount or £2,500, whichever is lower, on top of the levy itself becoming payable in full immediately.

Can CIL and Section 106 both apply to the same development?

Yes, but they are meant to fund different things. CIL is pooled by the charging authority towards its published list of local infrastructure priorities, while a Section 106 agreement is used for site-specific mitigation and affordable housing tied directly to the development in question. The same piece of infrastructure should not be funded twice through both mechanisms.

This article is general information, not personal tax or planning advice, and CIL charging schedules and reliefs vary by local authority and change over time. Your own position depends on facts we cannot see from here — please take advice before acting on anything above.

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