The short answer
Most basement conversions are charged the standard 20% rate of VAT, because work to an existing single dwelling is normally standard-rated. A reduced rate of 5% can apply in specific cases, for example where the work changes the number of dwellings (such as creating a self-contained flat) or where a property has been empty for two years or more before the work. Genuine new-build is zero-rated, but converting or extending your own home rarely qualifies. VAT can be a large hidden line on a basement budget, often adding thousands, so it is worth confirming the rate with a qualified accountant or directly with HMRC before you sign a contract. The rules are set by HMRC and depend on the nature of the work, not on who does it.
VAT is one of the most misunderstood costs on a basement project, and getting the rate wrong can be expensive. The sections below explain when the standard rate applies and the narrow cases where it does not.
VAT at a glance
- Standard work20% VAT
- Creating an extra dwellingPossibly 5%
- Empty 2+ yearsPossibly 5%
- New-build only0% (rarely applies)
- Confirm withHMRC / accountant
Why most basement work is standard-rated
VAT on building work follows rules set by HMRC, and the default position for work to an existing home is the standard 20% rate. A basement conversion that improves or extends your own house, adding a bedroom, study, living room or media room, is normally treated as ordinary alteration work and is fully VATable. This applies whether you convert an existing cellar or dig a new basement, because in both cases you are working on an existing single dwelling. The labour and most materials are charged at 20%, which is why VAT can add a substantial amount to an already large basement budget and should be allowed for from the outset.
| Situation | Likely VAT rate | Notes |
|---|---|---|
| Convert/extend own home | 20% | Standard alteration work |
| Create an extra dwelling | Possibly 5% | Changing number of dwellings |
| Property empty 2+ years | Possibly 5% | Renovating a long-empty home |
| Genuine new-build | 0% | Rarely applies to conversions |
Indicative guidance only. VAT treatment depends on HMRC rules and your specific circumstances.
The reduced and zero-rate cases
There are limited situations where a lower rate may apply. The 5% reduced rate can be available where the work changes the number of dwellings, for instance converting a basement into a self-contained flat so that one home becomes two, or where you are renovating a property that has been empty for two or more years. The zero rate is reserved for genuine new-build dwellings and certain works for disabled people, and rarely covers an ordinary conversion. These reliefs come with conditions and evidence requirements, so they are not something to assume. Where a project might qualify, the contractor charges the reduced rate directly, but only if the criteria are properly met and documented.
How VAT affects your budget and quotes
Because VAT is such a large proportion of a big spend, it is essential to know whether a quote is shown inclusive or exclusive of VAT. A figure quoted before VAT can look much lower than a comparable all-in price, so always check the basis when comparing contractors. For a six-figure basement project, the difference between an ex-VAT and an inc-VAT figure is significant, and it is a common cause of budget shock late in a job.
If your project genuinely qualifies for the reduced rate, the saving is worth pursuing properly. That means establishing the qualifying condition, such as the change in the number of dwellings or the period the property has been empty, gathering the evidence HMRC expects, and having the contractor apply the correct rate from the start rather than trying to reclaim later. For most homeowners improving their own house, though, the realistic planning assumption is the full 20%, and it is safer to budget on that basis and treat any reduction as a bonus.
Frequently asked questions
Can I reclaim VAT on a basement conversion?
Generally not for work on your own existing home, because there is no VAT reclaim scheme for ordinary conversions. The DIY housebuilders' scheme applies to new-builds and certain conversions of non-residential buildings, which a basement under an existing house does not usually meet.
When does the 5% VAT rate apply?
Mainly where the work changes the number of dwellings, such as creating a self-contained flat, or where a property has been empty for two or more years. The conditions are specific and evidenced, so confirm with HMRC or an accountant before relying on the lower rate.
Is a quote usually shown with or without VAT?
It varies by contractor, which is why you must check. A price shown before VAT looks lower than a comparable inclusive figure. For a large basement budget the difference is substantial, so always compare quotes on the same VAT basis.
Sources & further reading
- GOV.UK — VAT on building work and conversions (Notice 708)
- GOV.UK — reduced rate VAT for residential conversions
Figures on this page are typical UK ranges drawn from published sources and depend on your specific property. They are guidance, not a quotation.