— reviewed against the regs as they stand.
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 the document has to show
There is no prescribed form, which is why submissions vary so much. What every building control body is looking for is the same short set of facts:
- The dwelling it applies to — plot or house type, and which plots that type covers.
- The fittings schedule — each fitting, the figure entered for it, and its unit: flow rates in litres per minute, flush volumes and bath capacity in litres, white goods in litres per kilogram and litres per place setting.
- The calculated figure in litres per person per day, after the normalisation factor and the fixed external-use allowance have been applied.
- The target it is measured against — 125, or 110 where planning says so — and a clear pass.
- The date and who produced it.
That is the whole document. On a site with several house types you need one per type, and one type covers every identical plot.
When it’s due
Regulation 37 sets the backstop: the person carrying out the work must give notice of the calculated consumption to the local authority no later than five days after the work has been completed. Almost nobody should be working to that deadline, because the calculation depends on sanitaryware that gets specified and ordered months earlier. Most building control bodies ask for it with the plans or at first fix, and the sensible time to run it is before the bathroom order goes in — when a shower head is still a decision rather than a delivery.
How the calculation actually works
Each fitting is scored as capacity × use factor + fixed use. The use factors are published in Table 1 of the methodology and never change. The lines are added up, the total is multiplied by a 0.91 normalisation factor, and a fixed 5 litres per person per day of external use is added on the end.
An ordinary specification — a 6/4 dual-flush WC, 6 l/min basin taps, a 180 litre bath with a 10 l/min shower over it, an 8 l/min kitchen tap and unspecified white goods — lands at 123.9 l/p/d. Against a 125 target that is a pass with 1.1 litres to spare.
That margin is the thing worth understanding. A perfectly normal specification passes by barely a litre, so a late change — a bigger bath, a pumped shower, a waste disposal unit dropped into the kitchen design — can quietly turn a compliant scheme into a non-compliant one. The full arithmetic is set out in our no-spreadsheet walkthrough, fitting by fitting.
Why calculations get sent back
In rough order of how often it happens:
- The figures don’t match the fittings. A flow rate quoted from a brochure for a different pressure, or a spec that changed after the calculation was run.
- Multiple fittings averaged flat. Where several showers or taps of different flow rates are fitted, the methodology takes the higher of the arithmetic average and 70% of the highest-flow fitting — so one powerful master shower cannot be averaged away by a modest en-suite.
- The two adjustments missing. Leaving out the 0.91 normalisation or the 5 l/p/d external allowance changes the answer by more than most specification changes do.
- The wrong target. A planning condition required 110 and the calculation was run against 125. Check the decision notice, not the assumption.
- No plot schedule. A calculation for “House Type B” with nothing saying which plots are House Type B.
What it costs
Online services and assessors typically charge £25–£65 per calculation — and again for each different house type on the site. 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 +VAT, 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 to overflow, WC flush volumes, and your dishwasher and washing machine figures if they’re being specified. All of it is on the product spec sheets or the retailer’s listing. If you want to understand one line before you change it, start with shower flow rate — it’s usually the biggest number on the page.
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.
Common questions
What is a water efficiency calculation for Building Control?
A short compliance document showing that a new dwelling's calculated water use is no more than 125 litres per person per day — or 110 l/p/d where a planning condition imposes the tighter optional standard. It uses the official methodology, The Water Efficiency Calculator for New Dwellings, required by Approved Document G and Building Regulation 36. It is a paper exercise based on the fittings you are installing, not a measurement or a site visit.
When does Building Control need it?
Regulation 37 requires the person carrying out the work to give the local authority notice of the calculated consumption per person no later than five days after the work has been completed, using the methodology referenced in regulation 36. In practice most bodies want to see it far earlier — with the plans or before first fix — because the answer depends on sanitaryware that gets ordered early, and a late fail is an expensive one.
Which buildings need one?
New dwellings in England and Wales, including houses, flats, and dwellings formed by conversion or by splitting an existing building. Extensions to an existing dwelling do not need one, and neither do most non-domestic buildings, which are handled separately under Part G's non-domestic provisions.
Do I need to be qualified to produce it?
No. There is no accreditation attached to the water efficiency calculation the way there is to SAP. Anyone who can read a flow rate off a specification sheet can produce a compliant calculation, provided the methodology is applied correctly and the fittings entered are the fittings installed.
What if my calculation fails?
You change the specification, not the calculation. The shower is almost always the fastest fix — every litre per minute is worth about 4 l/p/d — followed by the part flush volume of the WC and then the basin taps. Rainwater or greywater harvesting can be credited too, but it is rarely needed just to reach 125.