Part G water calculations for conversions and change of use
Converting a barn, office, shop or house into new homes? Here is when a water efficiency calculation is needed and how to handle it.
At a glance
The Part G water efficiency requirement is not only for new-build houses. It also applies when a material change of use creates a dwelling, and that catches a lot of conversion projects.
Which projects need a water calculation
- New-build dwellings of any kind: houses, bungalows, flats and maisonettes.
- Buildings converted into a dwelling where they were not one before, for example barns, offices, shops, pubs, churches and commercial units.
- Buildings converted to contain a flat where they did not before, for example a house split into flats or a flat formed above a shop.
Each new dwelling created needs its own result against 125 litres per person per day, or 110 litres where a planning condition requires it.
Projects that usually do not
- Extensions and loft conversions to an existing house.
- Replacing a bathroom or kitchen in an existing home.
- Internal alterations that do not create a new dwelling.
Other parts of Part G, such as hot water safety, can still apply to that work. If you are unsure whether your project creates a new dwelling, check with your Building Control body early.
Prior approval conversions
Homes created under permitted development, such as office-to-residential conversions approved through prior approval, still need Building Regulations approval. The water efficiency calculation applies to them in the same way. Check the prior approval or planning documents for any condition requiring 110 litres.
Common issues on conversions
- Several units, one specification: a block of converted flats often uses the same fittings in each unit. Set it up as one house type and issue a report for each flat.
- Different flat sizes: the water calculation is per person, so the result does not depend on the number of bedrooms. Different fittings in different flats do change it, though.
- Low water pressure: in older buildings, record shower flow rates at the pressure actually available. That is often lower than the maximum quoted by the manufacturer.
Submitting it
As with a new build, Building Control needs a notice of the calculated consumption no later than five days after completion, and most want it before the final inspection. A report and calculation schedule for each dwelling covers this.
Frequently asked questions
Does a barn conversion need a Part G water calculation?
Yes, when the conversion creates a new dwelling. A material change of use that forms a dwelling must meet the same water efficiency limit as a new build.
Do house extensions need a water efficiency calculation?
Usually not. Extensions, loft conversions and bathroom refits to an existing home do not create a new dwelling, so the Part G water efficiency limit does not normally apply.
Do permitted development conversions need one?
Yes. Homes created under permitted development, such as office-to-residential conversions, still need Building Regulations approval, including the water efficiency calculation.
Calculating for a conversion?
Use the development option to set up each flat or unit type, check every result for free, and issue a report for each dwelling.
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