Asked for a water efficiency certificate? Here is what actually exists

Updated 21 September 2026 — reviewed against the regs as they stand.

Is there such a thing as a water efficiency certificate?

Not in the Building Regulations, no. Nothing in them requires or defines a water efficiency certificate, and no body is appointed to issue one. It is a piece of trade shorthand that has stuck — and because it has stuck, it is what most people search for and what most services sell.

What the law actually requires is a notice. Regulation 37 of the Building Regulations 2010 says the person carrying out the work must give the building control authority a notice specifying:

• which requirement applies — the 125 litres per person per day of regulation 36(2)(a), or the optional 110 l/p/d of regulation 36(2)(b); and
• the potential consumption of wholesome water per person per day for the completed dwelling.

Two numbers, in other words: the target and the result. Everything else — the fittings schedule, the arithmetic, the letterhead — exists to show that the second number is right.

So when somebody asks for a certificate, they are asking for the document that carries the calculation behind that notice. Nobody is being loose or misleading by calling it one; it just helps to know that the thing with legal force is the notice, and that no particular format is mandated for what sits behind it.

One wrinkle worth knowing: the notice goes to the building control authority, not the local authority. That wording changed on 1 October 2023 with the Building Safety Act reforms, which also added the higher-risk-building route below. A lot of guidance still in circulation — ours included, until recently — carries the pre-2023 text.

What the document has to contain

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.

Which dwellings need one

New dwellings in England and Wales: new builds, barn conversions, flats formed by splitting an existing building. An extension to an existing dwelling does not need one. Most non-domestic buildings do not either — Part G handles those separately.

If it is a dwelling and it did not exist before, assume you need the calculation. Sensible modern fittings usually pass comfortably — see how the 125 l/p/d figure works, or 110 where a planning condition applies.

When it is due

Regulation 37 sets the backstop: the notice must reach the building control authority no later than five days after the work has been completed. Where the dwelling is within a higher-risk building — at least 18 metres tall or at least seven storeys, and containing at least two residential units — the notice must instead accompany the application for a completion certificate.

Almost nobody should be working to either 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:

1. 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.
2. 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.
3. 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.
4. The wrong target. A planning condition required 110 and the calculation was run against 125. Check the decision notice, not the assumption — and if you want to know whether your area is likely to impose it, the Environment Agency’s water stressed areas classification lists all 23 water company areas.
5. 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 advertise roughly £20 to £95 for a single dwelling. The headline price is rarely the number that matters, though — what matters is what one purchase covers. Some are priced per dwelling, in which case a 40-plot site of one house type is forty purchases; others per dwelling type, where the same site is one.

Check which before you buy. On a repeated house type it is the difference between one fee and forty, for identical arithmetic.

Do it yourself, free

Our WaterMonkey water efficiency 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 is £25 +VAT, and that covers every identical plot on your site, not one plot. 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.