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Water Efficiency Calculations (Part G)

We’re regularly reminded that water is a resource to be used sensibly and if you’re building in an area particularly prone to drought, you may need to meet stricter water efficiency targets.

What does a Water Efficiency Calculation measure?

Building Regulations set out targets for the water efficiency of fittings and white goods in our homes. New dwellings in England and Wales must be designed to use no more than 125 litres of water per person, per day. In areas prone to low water supplies in the summer, the target can be reduced by the local authority to 110 litres.

The Water Efficiency Calculation looks at the flow rate of taps and showers, the flush size of toilets and the size of bath tubs. It also considers water used by dishwashers and washing machines, as well as taking into account rainwater harvesting systems.

Do I need a Water Efficiency Calculation?

Approved Document Part G (England and Wales) suggests a recommended list of flow rates for your design to help meet the above requirements. If you want to deviate from this recommendation, for example if you choose to instal an extra large bath tub and offset this with more efficient fittings elsewhere, a Water Efficiency Calculation will be required.

In Scotland, Section 3 covers water efficiency but does not specify water use targets.

Top Tips for Water Efficiency Calculations

  • You need to make sure that your design equals, or is better than, the water efficiency targets set by your Local Planning Authority to ensure compliance.

  • Although these calculations are needed on construction, it makes sense to complete them at the Design Stage.

  • If BREEAM, Code for Sustainable Homes (CSH) or Home Quality Mark (HQM) assessments also apply to your build, you will gain extra credits by hitting lower water use targets.

  • If you intend to instal extra large bathtubs, offset this with more efficient fittings elsewhere.

  • Using wastewater or rainwater systems can significantly improve the Water Efficiency result and achieve a Pass result. 

The Energist Approach

  • We will research the water use targets of your Local Planning Authority and assess your specification and plans to calculate the water use.

  • We will make recommendations and send your report and supporting documentation within your deadlines.

You may also need:

SAP   |   BREEAM   |   London Plan Energy Strategy

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