Establish which Part G water efficiency limit belongs in the brief
Read the planning decision and conditions alongside the project description. Planning requirements differ by council, and a general aspiration to save water is different from an imposed optional requirement. Approved Document G reproduces the regulation linking the optional requirement to planning permission. If the wording is uncertain, ask the project adviser to resolve it before the fittings schedule is treated as final.
The scope question matters for conversions as well as new construction. Approved Document G describes the dwelling creation and material-change-of-use circumstances relevant to its water-efficiency provisions. Do not assume every alteration to an existing bathroom triggers the same calculation exercise as creating a dwelling. Give the assessor a clear description of the works and the approval route.
Record the agreed target on the specification and calculation instruction. A purchaser should not have to search through planning correspondence to discover why a particular fitting was selected. Our flat conversion compliance checklist helps place water evidence alongside other conversion tasks without treating all conversions as identical.
This comparison uses Approved Document G, 2015 edition incorporating 2016 and 2024 amendments. Our water calculation service can assess the specified fittings against the requirement applying to the dwelling.
| Question | Standard requirement | Optional requirement |
|---|---|---|
| Consumption limit | 125 litres per person per day, measured using the approved methodology. | 110 litres per person per day, measured using the approved methodology. |
| When does it apply? | Where the water-efficiency requirement applies and the optional requirement has not been imposed. | Where planning permission specifies the optional requirement and makes compliance a condition. |
| Evidence to establish the target | Project scope and planning documents confirming which requirement applies. | The planning permission and condition that impose the optional requirement. |
| Calculation inputs | The specified sanitary fittings, appliances and relevant alternative water supplies. | The same categories of inputs, checked against the tighter overall consumption allowance. |
| Meaning of the result | An estimate of potential consumption, rather than a prediction of each resident's bill. | An estimate under the methodology, rather than a separate household meter allowance. |
| Design response | Select and record fittings that demonstrate the applicable requirement is met. | Coordinate the fittings schedule early so the tighter requirement is reflected in purchasing. |
Source: consumption limits from Approved Document G, 2015 edition incorporating 2016 and 2024 amendments: regulation 36(2) as printed in the document, paragraph 2.3 (125 or 110 litres per person per day) and paragraph 2.8 (the optional requirement of 110 litres per person per day).
Understand what the Part G water efficiency calculation measures
The calculation combines fitting and appliance data under a prescribed use model. It does not add together every manufacturer's flow rate as though all outlets run continuously. Approved Document G sets out the calculator methodology in its appendix and also describes a fittings-based alternative. Confirm the intended evidence route before preparing a schedule of isolated product claims.
A marketing description such as water saving is insufficient input. Record the exact product, its flow or capacity data and any component relied upon to achieve that performance. For a tap or shower, keep the relevant technical information with the product reference. For a bath, distinguish the capacity information required for the assessment from a general catalogue description.
The overall result is useful for comparing proposed schedules. If a preferred product changes the outcome, ask the assessor to test a complete alternative schedule. This allows the designer to consider user needs and product availability together rather than choosing a single very restrictive fitting without understanding its effect on the calculation.
Keep Part G water limits separate from actual resident consumption
A compliant estimate does not guarantee the same measured consumption after occupation. Residents' habits, leaks, maintenance and use of appliances affect actual demand. Explain the result as a design assessment so owners do not mistake it for a promise about their utility charges or a restriction enforced by a household meter.
Where alternative water sources form part of the proposal, Approved Document G says they should be reflected in the estimated use and addresses the associated supply provisions. Do not claim a benefit for a system that appears only as a possible future option. Identify what is actually specified, who designs it and what evidence will confirm the installed arrangement.
For a self-builder, a coordinated product list can prevent a late mismatch between the calculation and a showroom order. For several dwelling types, keep a matrix showing which schedule applies to each layout. The new-build compliance package can coordinate this evidence with energy, ventilation and overheating tasks; the new-build compliance checklist explains their project sequence.
Control substitutions against the Part G water efficiency limit
Ask the purchasing team to flag proposed replacements before ordering. A visually similar shower or bath may have different assessment inputs. Retain the superseded schedule, identify the changed product and obtain a revised calculation where needed. This leaves a clear record of the design decision rather than several conflicting versions circulating at completion.
Approved Document G addresses notification of the applicable requirement and potential consumption for the completed dwelling. Send our water calculation service the planning condition, final fittings schedule and supporting product data. Identify unresolved selections expressly so the report can distinguish confirmed inputs from items still awaiting a purchasing decision.