Free to model. Building Control certificate PDF £25 +VAT (£30 inc VAT).
Runs in your browser. Toggle the target between 125 and 110 l/p/d and watch the result update.
110 vs 125 — what's the difference?
Approved Document G sets 125 l/p/d as the baseline for every new dwelling. Local planning authorities in water-stressed areas can require the tighter optional 110 l/p/d standard, usually as a planning condition. Both figures include a fixed 5 l/p/d for external use, so the fittings have to come in at 105 l/p/d for the 110 standard versus 120 l/p/d for 125.
How to pass at 110 l/p/d
It's tighter but routine: efficient taps and showers, a dual-flush WC, a sensible bath size and a flow-limited kitchen tap usually do it, and rainwater or greywater reuse can be credited. WaterMonkey shows your fittings total live, so you can adjust a flow rate or a WC and watch the number drop under the target before you commit the specification.
Building in London? Your policy says 105 — it's the same number
London Plan Policy SI 5 requires residential development to achieve mains water consumption of 105 litres or less per person per day, excluding an allowance of up to five litres for external water consumption. Because the Part G calculation adds that fixed 5 l/p/d of external use at the very end, 105 excluding external and 110 including external are the same target. Set the calculator to 110, and a London scheme is satisfied.
The only thing worth doing deliberately is making the convention explicit on your submission, so a "105" in the planning file and a "110" in the Building Control file are obviously the same calculation. The separate proposed national cut to 105 l/p/d is a different figure again — that one includes external use, and it isn't in force.
More on London schemes — what to put on the submission, and the boroughs consulting on going tighter than the London Plan — on the London water efficiency calculation page.
Where the 15 litres come from
An ordinary specification lands at about 123.9 l/p/d, so you need roughly 14 litres. Three unremarkable changes usually do it: the shower from 10 to 8 l/min (worth 7.95 l/p/d), the WC from 6/4 to 4.5/3 (4.69), and the basin taps from 6 to 5 l/min (1.44). That lands the dwelling at 109.9 — a pass, with no change to the layout. The full route is set out here, and specifying the white goods is worth about another 5 l/p/d if they're part of the handover.
One dwelling type covers the estate
Building several plots to the same sanitaryware specification? One WaterMonkey calculation covers every identical plot, so a whole development doesn't need a separate calculation per house — see the main WaterMonkey calculator for multi-type schemes.
Frequently asked questions
When does the 110 litres standard apply instead of 125?
The 125 l/p/d standard in Approved Document G is the baseline for all new dwellings. The tighter optional 110 l/p/d standard applies only where a local planning authority has imposed it — usually through a planning condition in a designated water-stressed area. Always check your decision notice and conditions.
What is the fittings budget at 110 l/p/d?
Both figures include a fixed 5 l/p/d allowance for external water use, so the fittings-based part of the calculation has to come in at 105 l/p/d for the 110 standard (versus 120 l/p/d for the 125 standard). WaterMonkey shows your fittings total and the PASS/FAIL against whichever target you set, live.
How do I pass at 110 l/p/d?
It is tighter but very achievable: lower-flow taps and showers, a dual-flush WC, a smaller or efficient bath, and a flow-restricted kitchen tap usually get there. Rainwater or greywater reuse can also be credited. Model the fittings in WaterMonkey and watch the number until it passes.
I am building in London — my policy says 105, not 110. Which is it?
The same target, written a different way. London Plan Policy SI 5 requires mains water consumption of 105 litres or less per person per day excluding an allowance of up to five litres for external water use. The Part G calculation adds a fixed 5 l/p/d of external use at the end, so 105 excluding external and 110 including external are identical. Set WaterMonkey to 110 and a London scheme is satisfied.
Is the calculator free?
Yes — modelling the dwelling and seeing PASS/FAIL at 110 or 125 l/p/d is free in your browser. You only pay £25 +VAT (£30 inc VAT) to export the Building Control certificate PDF.
Back to the main Part G water calculator, or read what 125 l/p/d means and how the calculation reaches Building Control.