Worked CIL area calculation for a loft conversion
A loft conversion inside the existing roof usually creates no chargeable floorspace. Here is how the area is measured, which exemption applies and when to claim it.
Most loft conversions produce no Community Infrastructure Levy bill at all, because the levy is charged on new build floorspace and a conversion inside an existing roof creates almost none. The exception is a loft that becomes a separate flat, which counts as a new dwelling and is chargeable however small it is. Working out the CIL calculation for a loft conversion is therefore a two-stage job: decide how much new build gross internal area the scheme actually creates, then check which exemption applies and claim it in the right order.
Get the order wrong and the exemption is lost permanently. That is the part that catches people, far more often than the arithmetic.
Whether a loft conversion creates chargeable floorspace
The levy under the Community Infrastructure Levy Regulations 2010 applies to a chargeable development that either creates 100 m² or more of new build gross internal area, or creates one or more dwellings. Both tests look at net additional floorspace, so existing floorspace that is being retained and converted is deducted where the building has been in lawful use.
A loft conversion that fits inside the existing roof volume adds habitable space without adding a building. Rooflights do not change that. A rear dormer or a hip-to-gable change does add a small amount of enclosed volume beyond the old roof plane, and that is the part a charging authority may treat as new build.
The deduction for retained floorspace has a condition attached. The regulations only allow it where the 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. For a house you live in, that is never an issue. For a long-empty property bought at auction, it can be.
Measuring the area inside the roof
Gross internal area is measured to the internal face of the enclosing walls, floor by floor, including internal walls, stairwells and partitions. It is not the usable floor area a builder quotes you, and it is not the area with full standing headroom.
The treatment of restricted headroom under a roof slope is the one measurement question worth asking before you submit. Practice varies between charging authorities on whether space below about 1.5 m is counted, excluded or noted separately, and the difference on a small roof can be several square metres. Ask the CIL officer at the council how they measure a roof space, in writing, and keep the reply.
Measure from the drawings you are actually submitting. If the roof plan changes during the application, the area changes with it, and a later amendment under section 73 restarts the levy calculation on the new permission. The documents that go with that kind of amendment are set out in our section 73 application documents checklist.
The minor development threshold and the extension exemption
Two separate routes can take a householder loft conversion out of charge.
The first is the minor development provision. Where the chargeable development creates less than 100 m² of new build gross internal area and no new dwelling, no levy is payable. This one applies by operation of the regulations rather than by application, which is why most loft conversions generate nothing.
The second is the residential extension exemption, which sits with the self-build provisions in the regulations. It matters when the new build area creeps over 100 m², which a loft plus a large rear extension in the same application can do. The exemption has to be claimed, granted and recorded before any work starts, and the authority will expect the main dwelling to be your own home rather than a property you are improving to let or sell.
| Situation | New build area | New dwelling | Usual outcome |
|---|---|---|---|
| Loft inside existing roof, rooflights only | Nil or minimal | No | No levy payable |
| Loft with rear dormer | A few m² | No | Below the 100 m² threshold |
| Loft plus large rear and side extension | May exceed 100 m² | No | Claim the residential extension exemption |
| Loft converted to a self-contained flat | Any | Yes | Chargeable, with a deduction for retained floorspace |
The paperwork sequence that keeps the exemption
The sequence is unforgiving and the regulations do not allow the authority much discretion to forgive a late claim.
- Submit the additional information form with the planning application so the authority can work out whether the development is liable.
- Assume liability in writing on the assumption of liability form, in the name of the person who will own the land when work starts.
- Claim the exemption on the residential extension exemption claim form, and wait for the authority's written decision granting it.
- Serve the commencement notice at least one day before work begins, quoting the planning permission and the intended start date.
- Keep all four acknowledgements with the building contract.
Starting work before the exemption has been granted, or failing to serve a commencement notice, can remove the exemption and make the full amount payable. The forms carry numbers on the Planning Portal, with the commencement notice and the residential extension exemption claim among the most commonly missed; check the current numbering before you fill anything in, because the set has been revised more than once.
An illustrative calculation
This worked example is illustrative only, using round figures for a typical 1930s semi-detached house.
Take an existing roof with an internal footprint at the new floor level of 8.0 m by 5.2 m, so 41.6 m² of converted space. All of it sits inside the existing roof envelope and is retained floorspace rather than new build. A rear box dormer projects beyond the old roof plane and encloses a further 7.4 m² measured at floor level.
New build gross internal area is therefore 7.4 m², not 49.0 m². No new dwelling is created. The scheme falls below the 100 m² minor development threshold and no levy is payable, so no exemption claim is needed at all.
Now change one fact. The same owner adds a single-storey rear extension of 46 m² and a garage conversion counted as retained floorspace. New build area becomes 53.4 m², still under the threshold. Add a two-storey side extension of 52 m² and the total reaches 105.4 m², which crosses the line and makes the exemption claim essential. The method of working from a policy figure to a submitted number is similar to the approach in our cycle parking calculation from a policy standard.
Where a liability notice still lands
A liability notice is not a bill. The charging authority issues one for any development it treats as liable, and it will often issue one showing a zero or a nil chargeable amount. Read it rather than filing it.
Notices also arrive where the authority has measured the roof differently, has missed the retained floorspace deduction, or has treated a dormer as a new storey. And on a loft that becomes a separate flat, the notice will show a figure calculated from the local charging schedule rate for the relevant zone, index-linked, with the retained floorspace deducted. Charging schedule rates are set locally and indexed each year, so check the current schedule on the charging authority's own website rather than relying on a figure quoted in a builder's estimate.
Checking and challenging the figure
Three things are worth checking line by line on any notice: the floor areas used, the deduction for retained in-use floorspace, and the indexation.
If the chargeable amount looks wrong, ask the charging authority for a review of the calculation within 28 days of the liability notice being issued. If the review does not resolve it, the regulations provide an appeal to the Valuation Office Agency on the chargeable amount, and separate appeal routes exist for surcharges and for the refusal of an exemption. Those routes have short deadlines and no discretion to extend, which is a recurring theme with this levy. The same discipline applied to a measured planning calculation shows up in our canopy cover percentage worked example.
If you are putting the application together yourself, the supporting documents matter as much as the forms, and the Planning Statement is usually the place where the floorspace figures are explained to the case officer.
If you are still working out which supporting documents the council will ask for, enter your postcode on PlanScope to see which reports the site is likely to need.
This is general guidance, not legal advice. Check the position with your local planning 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.
Frequently asked questions
Is CIL payable on a loft conversion?
Usually not. The levy is charged on net additional new build gross internal area, and a conversion inside an existing roof creates very little of it. Unless the work creates a separate dwelling, most loft conversions fall below the 100 m2 minor development threshold in the Community Infrastructure Levy Regulations 2010 and nothing is payable.
Does a rear dormer count as new floorspace?
The part of a dormer that projects beyond the original roof plane encloses volume that did not exist before, so a charging authority may count it as new build. On a typical box dormer that is a handful of square metres. It rarely matters on its own, but it adds to other extensions in the same permission.
When must the residential extension exemption be claimed?
Before any work starts. You assume liability in writing, submit the exemption claim, wait for the authority to grant it in writing, then serve a commencement notice before beginning. Starting work before the exemption is granted, or failing to serve the commencement notice, can remove the exemption entirely with no discretion to restore it.
How is gross internal area measured in a roof space?
To the internal face of the enclosing walls at floor level, including internal partitions and the stairwell. The treatment of space with restricted headroom under the roof slope varies between charging authorities, so ask the CIL officer in writing how they measure a loft before you submit drawings, and keep the answer on file.
What if the liability notice figure looks wrong?
Ask the charging authority to review the calculation within 28 days of the notice being issued. If that does not resolve it, the regulations allow an appeal to the Valuation Office Agency on the chargeable amount, with separate routes for surcharges and refused exemptions. All of these deadlines are short and cannot be extended.
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.