England and Wales
Extensions and the council tax band
This page is for someone planning a loft conversion, an extension, an annexe, or a garage conversion. It follows two public GOV.UK pages. It is not a decision about your home, and the Valuation Office will not advise on a plan before the work exists.
A larger home, and the next purchase
Council Tax band changes says a property may move to a higher or lower band because its size or internal structure has been altered. One of the cases it lists is that the property has been increased in size. The published line for that case is:
A property that's increased in size may move to a higher band when it's next purchased.
The assessment page, How domestic properties are assessed for Council Tax bands, says the same thing:
If a property has increased in size since it was first assessed, it may move to a higher band when it is next sold.
That is the rule for an extension that makes the same home larger, including a loft that is another room in the house, or a garage brought into the same home. GOV.UK describes the band review at the next purchase or sale, not as an immediate change while you still own it. You do not report that sale yourself:
You do not need to tell us about sales, as this information comes to us from the purchaser's solicitor via the Stamp Duty Land Tax Office.
When it can change without a sale
The band-changes page gives this example:
The band can change without a sale in some situations. For example, on the creation of a self-contained annexe.
The assessment page puts it this way:
In some situations a banding can be changed without a sale. For example, if a property is extended with a self-contained annexe.
By law, any building or part of a building which has been constructed or adapted for use as separate living accommodation must have its own banding.
A self-contained annexe, or a garage conversion built as its own living space, is the case those sentences are about. The assessment page says the decision looks at physical features such as its own sleeping space, somewhere to prepare food, washing facilities and a toilet, and its own access. How the space is used does not decide it. Any increase is payable from the date the list changes:
Any increase in the Council Tax you pay will apply from the date that the Council Tax valuation list is altered.
If the Valuation Office tells you the band has changed, the band-changes page says you can appeal within 6 months of that notice. It is always free to challenge a band with the Valuation Office. A challenge can move a band up.
Check the current band, then the evidence pack
The free check compares the current band with similar sales. It does not predict how the Valuation Office will treat planned work. If the result looks high, the same page offers the evidence pack (£4.99) and the evidence pack and appeal guide (£9.99). You send any challenge yourself.