What they’re asking for
Every new dwelling in England and Wales — new builds, barn conversions, flat splits — must show that its calculated water use won’t exceed 125 litres per person per day (or 110 in some areas). It’s Part G of the Building Regulations, and the document Building Control wants is a short report using the official methodology: The Water Efficiency Calculator for New Dwellings.
It is not a measurement or a site visit. It’s a paper exercise based on the fittings you’re installing: how much water your taps and showers flow, how big the bath is, how your WC flushes. Sensible modern fittings usually pass comfortably — see how the 125 l/p/d figure works.
What it costs
Online services and assessors typically charge £25–£65 per calculation. There’s nothing wrong with that — but the calculation itself is well within reach of anyone who can read a tap’s specification sheet.
Do it yourself, free
Our WaterMonkey calculator runs the full official methodology in your browser. Enter your fittings, watch the PASS/FAIL update live, and adjust until you’re under target — all free. When you’re happy, the Building Control-ready PDF report is £25, and that covers every identical plot on your site. You only pay when it’s right.
What you’ll need to hand: the flow rates (litres/minute) of your taps and showers, bath capacity, WC flush volumes, and your dishwasher/washing machine details if specified. All of it is on the product spec sheets or the retailer’s listing.
Rather hand it off?
If you’d prefer it done for you — or your case is complicated by rainwater harvesting, greywater reuse or a tight planning condition — our Part G service starts from a simple email with your sanitaryware list. Either way, the calculation shouldn’t hold up your sign-off.