Does a Garden Gym or Garage Extension Trigger the Community Infrastructure Levy? The 100 Square Metre Rule and Exemptions in England

Building a home gym outside the house raises a cost question that rarely appears in planning guides: the Community Infrastructure Levy (CIL), a charge some councils place on new floor space. For most garden gyms and garage extensions the answer is that nothing is payable, but the reasons matter, and a larger conversion or a build done first and regularised later can change the answer. This guide summarises GOV.UK’s planning practice guidance on the levy, which was last updated on 17 June 2026 and is written for England, and the wording of the CIL Regulations 2010 on legislation.gov.uk, whose relevant provisions are marked as applying to England and Wales. Scotland and Northern Ireland are not covered by these sources. It is general information, not legal advice.

What CIL is and where it applies

The guidance describes the levy as a charge that local authorities can place on new development in their area, to help deliver infrastructure. It says the levy only applies where a local authority has consulted on and approved a charging schedule setting its rates and published it on its website. Most new development that creates net additional floor space of 100 square metres or more, or a new dwelling, is potentially liable. Because each authority decides whether to charge, a homeowner needs to check the charging schedule for the council that covers their address.

The 100 square metre rule and a garden gym

Regulation 42 of the CIL Regulations 2010 says liability does not arise where, on completion, the gross internal area of new build on the relevant land will be less than 100 square metres. The exemption does not apply where the development comprises one or more dwellings. The regulation defines “new build” as the part of the development that comprises new buildings and enlargements to existing buildings. The GOV.UK guidance repeats the point: minor development with a gross internal area under 100 square metres is generally exempt, unless it results in a new dwelling.

A single-room garden gym is far below that size, so it falls within the minor development exemption. The regulation looks at the total new building or enlargement on the relevant land, so a gym room added alongside a larger extension counts towards the 100 square metres.

Buildings that never pay: “people do not normally go”

The guidance also lists buildings that do not pay the levy at all, and regulation 6(2) describes two kinds: a building into which people do not normally go, and a building into which people go only intermittently to inspect or maintain fixed plant or machinery. A storage shed for kit can be in that category, but a gym is a place people go to train, so it belongs in the ordinary size test above, not this exclusion.

Extensions and annexes

The guidance says people who extend their own homes or build residential annexes in the grounds of their own homes are exempt provided the criteria in regulations 42A and 42B are met. The main dwelling must be the person’s principal residence and they must have a material interest in it. An extension is exempt if it enlarges the principal residence and does not comprise an additional dwelling. The guidance adds that residential extensions under 100 square metres that are not part of a development creating a new dwelling are already exempt under the minor development exemption. A gym is not a dwelling, so the annex exemption, which is for a new dwelling within the curtilage, is unlikely to apply to it.

For any exemption, a claim must be submitted to the collecting authority before development commences, using the forms named in the guidance: Form 8 for an annex and Form 9 for an extension. The guidance also says an annex exemption requires a commencement notice (Form 6) before work starts, and that failing to submit one on time results in a surcharge. Elsewhere the guidance sets that surcharge at 20% of the notional chargeable amount, capped at £2,500. An annex exemption can be withdrawn if the annex is let or sold separately within 3 years of completion.

Building first and asking later

The guidance includes a warning for anyone tempted to build first. Developments granted retrospective permission under section 73A of the Town and Country Planning Act 1990 cannot obtain relief or exemption from the levy, other than the minor development exemption, because an exemption must be granted before development commences, and a section 73A permission is treated for CIL purposes as commenced when granted. For a small gym that limits the impact, since the minor exemption remains, but for a larger building it means the annex and extension exemptions are lost. The guide to certificates of lawfulness and the guide to permitted development for a garden gym cover how to check the planning position before starting.

Where a home gym might reach the threshold

• A large detached building with several rooms, or a substantial two-storey outbuilding, if the total new gross internal area reaches 100 square metres.
• A garage conversion combined with an extension, if the enlargement pushes the new build past the threshold. The guide to garage conversions covers the regulations side.
• Any scheme that creates a new dwelling, where the size threshold does not apply.

The bottom line

In England, and in England and Wales for the CIL Regulations 2010, the levy only applies where the council has an approved charging schedule, and new build under 100 square metres is exempt unless it includes a dwelling. A single-room garden gym is well below the limit. Extension and annex exemptions must be claimed before work begins, retrospective permission under section 73A removes those exemptions, and larger schemes should check the council’s schedule first. Scotland and Northern Ireland fall outside these sources.

Sources

• Ministry of Housing, Communities and Local Government, GOV.UK, “Community Infrastructure Levy”
• The Community Infrastructure Levy Regulations 2010, regulation 42, “Exemption for minor development”
• The Community Infrastructure Levy Regulations 2010, regulation 6, “Meaning of development”