Water efficiency calculation for Building Control: what to submit
What Building Control needs to see for Part G water efficiency, when they need it, and how to avoid a delay at completion.
At a glance
For every new dwelling, and every dwelling created by a material change of use, Building Control needs evidence that the home meets the Part G water efficiency limit. It is a small part of the job, but a missing water calculation is a common reason for a completion certificate being held up.
What Building Control asks for
- The calculated water consumption in litres per person per day, worked out using the method in Appendix A of Approved Document G.
- The requirement it is measured against: 125 litres, or 110 litres where a planning condition imposes the optional requirement.
- The specification behind it: the flush volumes, flow rates, bath capacity and appliance figures used, so the surveyor can check them against what has been installed.
A summary report plus a full calculation schedule covers all three. Building Control surveyors and approved inspectors check the inputs against the installed fittings, so keep manufacturers' data sheets for the products you specify.
When it has to be submitted
Under Regulation 37, the person carrying out the work must give Building Control a notice of the dwelling's calculated water consumption no later than five days after the work is completed. In practice most Building Control bodies want it before the completion inspection, and many ask for it at plan stage so the specification can be agreed early.
If fittings change on site
Substitutions happen. If a different shower, bath or WC is fitted, the calculation must reflect what was actually installed. Re-run it with the new figures and issue an updated report, otherwise the figures will not match what the surveyor sees.
Developments with several house types
On a multi-plot site, each dwelling needs its own result. Plots that share a house type and specification will give the same figure, but Building Control still expects a document for each plot. Our calculator lets you set up house types once and issue a report for every plot, at £10 + VAT for each additional plot.
What our report package includes
- A Water Efficiency Report showing the property, the requirement and the final figure.
- A Part G Calculation Schedule listing every fitting, the Appendix A factors and each contribution.
- A unique verification code on every document, so Building Control can confirm it is genuine.
Reports are only issued for a passing specification, and the final decision always rests with your Building Control body.
Frequently asked questions
When must the water calculation be given to Building Control?
Regulation 37 requires a notice of the calculated water consumption no later than five days after the work is completed. Most Building Control bodies prefer to see it before the completion inspection.
Does every plot need its own water efficiency report?
Yes. Each dwelling needs its own result. Plots that share a house type and specification will have the same figure, but Building Control still expects a document for each plot.
What happens if different fittings are installed on site?
Recalculate with the fittings that were actually installed and issue an updated report, so the figures match what the surveyor inspects.
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