A water calculation needs the actual proposed fixtures and appliance information. A broad product range, a sales description or an early allowance is not enough once procurement changes begin.
Approved Document G gives guidance on water efficiency for new dwellings. Its standard calculation route uses estimated consumption across the whole dwelling. That is why an individual tap or shower figure does not answer the project question by itself. The design team needs one consistent fixture schedule and a record of any change before products are ordered or installed.
Start with the project target
Approved Document G describes 125 litres per person per day as the standard for new dwellings, including outdoor use, with an optional limit of 110 litres per person per day where imposed through planning. Planning policy, employer requirements or a project condition can create a different target. Record the applicable target before comparing products.
Use exact product information
Collect the manufacturer data for the precise WC, basin tap, bath, shower, dishwasher and washing machine proposed. Product families can contain several flow rates and capacities. A schedule that says only the brand or range leaves too much open to change.
Control substitutions
A substitution can change the calculation even when the replacement appears similar. Give procurement and site teams a simple rule: if the sanitaryware, tap, shower, appliance or water-reuse proposal changes, send the exact data for review before it is fixed.
Keep the calculation and installation aligned
The final record should identify the dwelling type, calculation target, fixture schedule and any approved changes. This makes it easier to resolve a building-control query and avoids trying to reconstruct product information after handover.
Confirm the required target
Check the building-control and planning brief for the water-efficiency target that applies to the dwelling.
Build a precise fixture schedule
List manufacturer, product reference and the performance data used for every relevant fixture and appliance.
Review changes before ordering
Treat substitutions as a design change, with the product data checked against the project calculation.
File the final evidence
Keep the completed schedule and calculation with the relevant dwelling and completion documents.
Frequently asked questions
Is a low-flow shower enough to show that a dwelling meets the target?
No. The calculation considers the whole dwelling and the applicable method. The other fixtures, appliances and any alternative water source can affect the result.
What does the 125 litres per person per day figure mean?
Approved Document G sets out 125 litres per person per day as the standard for new dwellings. The optional requirement is 110 litres per person per day where imposed through planning. Both include the outdoor-use allowance. Confirm the target for the actual project before selecting products.
When should a fixture substitution be checked?
Before it is ordered or installed. Keep the precise manufacturer data with the change request so the calculation and the final installation stay aligned.