Water Efficiency Calculations
Frequently Asked Questions

A water efficiency calculation predicts how much water each person in your new home will use per day, based on the toilets, taps, showers, baths and appliances you're planning to fit, and checks that figure against the limit set by Part G (Requirement G2) of the Building Regulations.

It's a paper-based exercise carried out using standardised consumption figures for each fitting — not a meter reading of actual use — added together under the Government's Water Efficiency Calculator for New Dwellings, the approved national calculation methodology.

It's a genuine condition of building control sign-off under Regulation 36: every new dwelling needs a compliant calculation submitted before a completion certificate can be issued, so it's not optional paperwork.

Yes, if your project involves a new dwelling — a new-build house or flat, or a conversion that creates a new, separate dwelling — you'll need a compliant calculation submitted to building control before completion.

A straightforward extension to your existing home, where no new dwelling is created, doesn't normally trigger this requirement.

If you're not sure whether your project counts as creating a new dwelling — for example an annexe, a converted outbuilding, or splitting a house into multiple units — it's worth checking early rather than assuming. Get in touch and we can confirm whether it applies to your project.

125 litres per person per day is the national mandatory limit that applies by default. The tighter 110 litre limit only applies where your planning permission specifically requires it as an "optional requirement" condition — it isn't automatic.

Local planning authorities in water-stressed areas (and some with their own local plan policies) commonly attach the 110 litre condition to permissions in their area. Whether it applies to you depends on your planning decision notice, not on where you happen to be building in general terms.

This matters in practice because designing to 125 litres when your planning consent actually requires 110 can hold up discharge of that planning condition, even once building control itself is satisfied. If you're not sure which target applies to your project, get in touch and we'll check your planning conditions for you.

It doesn't necessarily mean your bathroom or kitchen fittings need to change. Choosing taps, showers or other fittings with a higher flow rate than Part G allows is a common situation, and it's usually something we can resolve within the assessment itself.

In many cases, a simple pipework adjustment on the supply to that fitting brings its practical flow rate down to a compliant level, without needing to swap the fitting itself for a lower-spec alternative. This means you can often keep the products you actually want.

In cases where the shortfall is larger, other adjustments (such as a lower-capacity bath, a more efficient WC, or in some cases rainwater/greywater recovery) may be needed instead. Get in touch if your calculation is currently coming out non-compliant and we'll talk through the most cost-effective way to bring it into line.

In order to complete a water efficiency calculation, we'll generally need:

• Full plans showing every room where water is used — bathrooms, en-suites, kitchen, utility, and any other wet rooms.
• The exact make and model of each fitting being specified (WCs, taps, showers, baths, washing machine, dishwasher), since consumption figures are taken from each product's own published data.
• Details of any water-saving measures, such as rainwater harvesting or greywater recovery, if included in the design.

If your final fittings haven't been chosen yet, we can run the calculation against representative/typical products first so you know what standard you're aiming for, then confirm it once your actual specification is finalised.

Ideally once your bathroom, kitchen and utility fittings are specified — or at least provisionally chosen — but before anything is ordered or installed.

Doing it at this stage means that if any adjustment is needed, it's a simple change on paper (for example, a minor pipework adjustment) rather than a physical rework after fittings are already in place.

That said, it can also be run earlier using typical/generic fittings if you want an early steer on what standard your project needs to hit, and then finalised later. Get an instant quote whenever suits your programme.

Yes — the government has confirmed plans, following a 2025 consultation, to tighten the national standard from 125 to 105 litres per person per day, with the optional stricter standard dropping from 110 to 100 litres per person per day.

This isn't in force yet. It's still subject to final parliamentary approval, and is expected to apply later this year with a six-month transitional period once it does — so projects going through building control today are still assessed against the current 125/110 figures.

It's worth being aware of if your project won't reach building control for some time, since the standard you're designing to may change before then. Get in touch if you'd like us to confirm which standard applies to your particular timeline.