Worked levy calculation for an extension with a garage
Most householder extensions fall outside the levy because of the 100 square metre minor development rule. An attached garage is exactly what pushes a scheme over it.
Most householder extensions pay no Community Infrastructure Levy, because the minor development exemption in the Community Infrastructure Levy Regulations 2010 removes the charge where the new build gross internal area is less than 100 m2 and no new dwelling is created. Add a garage and that headroom disappears faster than people expect, because a garage counts as floorspace whether or not anyone will ever live in it.
This piece works through the measurement, the threshold and the exemption process, with an illustrative calculation at the end. Every figure in that calculation is a round number chosen to show the arithmetic, and none of them is any particular council's rate.
Measuring gross internal area correctly
The levy is charged on gross internal area, not on footprint and not on external dimensions.
Gross internal area is measured to the internal face of the external walls, and it includes internal partitions, internal structural walls, stairwells, lift shafts, service risers and any space with the headroom to be usable. It is measured on every floor, so a two storey extension is measured twice.
Three habits produce a defensible figure.
- Measure from the proposed plans at a stated scale, or better, from dimensioned drawings, and state which drawing and revision you used.
- Measure floor by floor and show the areas separately before totalling them.
- Note every exclusion you have applied and why, because the charging authority will check the exclusions rather than the additions.
Where the measurement is arguable, the authority's own calculation governs unless you challenge it, so submit your figure with the working shown rather than a single number.
What counts as new build floorspace
New build means floorspace that did not exist before. Two consequences follow.
Retained floorspace can be deducted where the existing building has been in lawful use for a continuous period of at least six months within the three years ending on the day planning permission first permits the development. That matters where an extension replaces a demolished lean-to or where a garage is being rebuilt, and the deduction is often overlooked on householder schemes.
Where the existing building has been empty for longer than the regulations allow, the deduction is not available, and a replacement extension can be charged on its full area. Check occupation history before assuming the offset applies.
Conversions of existing space, such as turning a loft into a bedroom without raising the roof, create no new floorspace and so create no levy liability. A loft conversion that adds a dormer adds only the floorspace that the dormer creates.
Whether the garage is included
Yes, in almost all cases, and this is the point of the article.
An attached garage enclosed within the external walls of the extension is part of the gross internal area of the building. A detached garage is a separate building, and its gross internal area is still new build floorspace for levy purposes. A carport open on two or more sides is arguable, and the charging authority's view should be sought rather than assumed.
Whether the floorspace is charged, and at what rate, depends on the charging schedule. Most adopted schedules set a rate for residential development and treat ancillary domestic floorspace as residential. A minority set a nil rate for particular categories. Read the schedule for the relevant zone rather than relying on a summary, because charging schedules are zoned and rates differ across a single authority.
Applying the charging schedule rate and index
The chargeable amount is calculated using the formula in the Regulations, which multiplies the rate in the charging schedule by the chargeable area and then applies an index adjustment. The index brings the rate from the year the charging schedule took effect to the year the permission was granted.
The index used is the levy index published by the Royal Institution of Chartered Surveyors for this purpose, which replaced the earlier tender price index. So a scheme permitted in one year, under a schedule adopted several years earlier, carries a higher effective rate than the headline figure suggests.
Practically, that means three numbers before you can calculate anything: the rate for your zone and development type, the index figure for the year the schedule took effect, and the index figure for the year permission is granted. The charging authority publishes the first and will confirm the other two.
The residential extension exemption and its limits
There are two separate routes out of a charge, and they work differently.
| Route | How it works |
|---|---|
| Minor development exemption | Applies automatically where new build gross internal area is under 100 m2 and no new dwelling is created. Nothing to claim. |
| Residential extension exemption | Must be claimed and acknowledged before development starts. Available to an owner who occupies the dwelling as their sole or main residence. |
The residential extension exemption is the one that requires care, because it is procedural. You must own the dwelling, occupy it as your principal residence, claim the exemption, receive the authority's acknowledgement, and only then start work. Starting before the acknowledgement arrives loses the exemption, and there is no discretion to restore it.
That trap catches self-managing homeowners more than anyone else. A builder with a free week in March starts the groundworks, and a £10,000 liability crystallises that nobody intended.
Forms and their order
The order matters more than the paperwork.
- Submit the levy additional information form with the planning application, giving the existing and proposed floor areas.
- Once permission is granted, assume liability by submitting the assumption of liability form.
- Submit the residential extension exemption claim, where you are relying on it, and wait for the authority's written acknowledgement.
- Submit the commencement notice, giving the date work will start, and keep the acknowledgement.
- Start work only after the notice has been served and any exemption acknowledged.
Forms are numbered on the Planning Portal and on the charging authority's website, and the numbering has changed over the years, so work from the current set rather than a saved copy. Serve everything in writing and keep the acknowledgements together with the decision notice.
The levy is a liability on the land, enforced through surcharges, stop notices and ultimately a charge on the property. It is not a fee that can be negotiated after the event.
Illustrative figures for a rear extension
The following is illustrative only. The rate is a round number chosen for the arithmetic and does not reflect any council's schedule.
Take a 1930s semi-detached house with a single storey rear extension of 38 m2 gross internal area and an attached single garage of 17 m2. New build floorspace totals 55 m2, no new dwelling is created, and the minor development exemption applies automatically. The levy is nil, and no exemption claim is needed.
Now change the scheme. A two storey rear and side extension of 82 m2 across both floors, plus a detached double garage of 32 m2, gives 114 m2 of new build floorspace. That exceeds the 100 m2 threshold, so the minor development exemption does not apply. At an illustrative rate of £100 per m2, before indexation, the charge on 114 m2 would be £11,400, and indexation would increase it. The residential extension exemption is then the route to nil, provided the owner occupies the house as their principal residence and claims it correctly before starting.
The lesson is in the second example. Nothing about the garage changed the family's living space, and it moved the scheme across the threshold on its own.
Where the extension is also in the Green Belt or another constrained location, the planning case matters as much as the levy, and our outline for a planning statement for a Green Belt householder extension covers that. A short Planning Statement is worth including with any householder application where policy is finely balanced, and PlanScope pricing sets out the cost of producing one as a draft.
This is general guidance, not legal advice; check the position with your charging authority or a qualified professional before relying on it. Levy rates and index figures change, so check the current charging schedule. PlanScope provides AI-assisted draft reports for planning applications. Reports should be reviewed before submission. We do not guarantee planning approval.
Frequently asked questions
Do householder extensions pay the Community Infrastructure Levy?
Usually not. The minor development exemption in the Community Infrastructure Levy Regulations 2010 removes the charge automatically where the new build gross internal area is less than 100 square metres and no new dwelling is created. Above that threshold a charge arises unless the residential extension exemption is claimed and acknowledged before work starts.
Is a garage included in the levy calculation?
Yes in almost all cases. An attached garage inside the external walls forms part of the gross internal area of the building, and a detached garage is a separate building whose gross internal area is still new build floorspace. A carport open on two or more sides is arguable, so ask the charging authority rather than assuming.
How is gross internal area measured for the levy?
To the internal face of the external walls, on every floor, including internal partitions, structural walls, stairwells and service risers. Measure floor by floor from dimensioned drawings, state the drawing and revision used, and note every exclusion you have applied, because the authority will scrutinise the exclusions rather than the additions.
What happens if I start work before claiming the extension exemption?
You lose it. The residential extension exemption must be claimed and acknowledged in writing by the charging authority before development starts, and there is no discretion to restore it afterwards. Serve the assumption of liability, the exemption claim and the commencement notice in order, and keep every acknowledgement with the decision notice.
Can I deduct the floorspace I am demolishing?
Sometimes. Retained or demolished floorspace can be offset where the existing building has been in lawful use for a continuous period of at least six months within the three years ending on the day permission first permits the development. Where the building has been empty longer than that, the deduction is not available, so check the occupation history.
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.