Water Efficiency Calculations
Part G Optional Requirement Compliance
Water Efficiency Calculations — Part G Optional Requirement Compliance
Water efficiency calculations are required for residential developments where the local planning authority has imposed a planning condition requiring compliance with the optional requirement for a maximum water consumption of 125 litres per person per day under Part G of the Building Regulations. This condition is commonplace across much of the South East and East of England, areas that the Environment Agency designates as being under serious water stress, and is also increasingly being applied in other parts of England where local planning policies seek to reduce per capita water demand from new development.
Failing to discharge a water efficiency planning condition is not a minor administrative matter. It prevents the local planning authority from issuing a certificate of compliance with planning conditions, which in turn prevents legal completion in most cases. Developers who leave water efficiency calculations to the last moment frequently find themselves scrambling to substitute compliant fittings for non-compliant ones at a late stage in the build, incurring additional costs and risking programme delays. Engaging SES at the design stage for water efficiency calculations ensures that the sanitary fittings, kitchen taps, showers, and white goods specified in the development are confirmed as compliant before procurement is finalised.
What Is the 110 Litre Optional Requirement?
The Building Regulations Part G sets a default standard for water consumption in new residential dwellings of 125 litres per person per day. Local planning authorities in areas of water stress may apply the optional requirement, which reduces this target to 110 litres per person per day. The optional requirement is typically expressed in the planning decision notice as a pre-commencement or pre-occupation condition, requiring the developer to demonstrate compliance with the 110 litre standard before work begins or before the dwelling is occupied.
The 110 litre figure represents the total expected daily water use per person across all water-consuming activities in the dwelling, flushing the toilet, showering, bathing, washing hands, washing dishes, doing the laundry, and drinking and cooking. Achieving this target requires careful selection of water-efficient fittings across all of these categories, and in practice it typically means specifying a dual-flush WC with a reduced flush volume of six and four litres or less, a flow-restricted shower head or thermostatic shower valve with a flow rate at or below eight litres per minute, a water-efficient basin mixer tap with a flow rate of no more than six litres per minute, and a kitchen tap with a similar flow restriction.
How the Calculation Works
The water efficiency calculation uses a spreadsheet-based assessment tool that models the expected daily water consumption of the dwelling based on the number of bedrooms, which is used as a proxy for the number of occupants, and the specifications of all water-consuming fittings and appliances. The specification data for each fitting is entered into the calculator and the expected daily contribution of that fitting to the total per-person consumption is determined based on published consumption factors for each end use category.
The calculator produces a total predicted daily consumption in litres per person per day, which must be at or below 110 to satisfy the optional requirement. The calculation also distinguishes between potable water consumption (water treated to drinking quality standard) and total water consumption, and some planning conditions specifically require the total consumption, including rainwater harvesting or greywater recycling contributions, to be reported alongside the potable consumption figure.
Design Stage and As-Built Calculations
Water efficiency calculations can be prepared at two stages. The design stage calculation is based on the proposed fittings specification and is typically submitted to the local planning authority as part of the pre-commencement discharge of conditions process, confirming that the specified fittings are capable of achieving the required standard. The as-built calculation is prepared after construction is complete, confirming that the fittings actually installed match the design specification and that the 110 litre target is achieved in the completed dwelling.
Most planning conditions require both a design stage calculation confirming the specification and an as-built calculation confirming installation. SES provides both stages as standard for our water efficiency calculation service.
What If the Initial Specification Fails the Calculation?
If the initial fittings specification entered into the water efficiency calculator produces a result above 110 litres per person per day, SES will identify which fittings are making the largest contribution to the excess and advise on alternative products that would bring the calculation into compliance. In most cases, the adjustments required are minor, substituting a standard-flow shower head for a flow-restricted alternative, for example, or upgrading a WC flush volume from nine and four litres to six and four litres, and do not significantly increase the cost of the development.
The time to identify these issues is at the design stage, before fittings have been procured and installed. A developer who discovers at practical completion that the installed fittings do not achieve the 110 litre target faces either the expense of replacing compliant fittings throughout the development or the cost and delay of renegotiating the planning condition with the local planning authority. Neither outcome is desirable, and both are entirely avoidable with early engagement.
SES are a multi-disciplined environmental consultancy with a proven track record of providing high quality.
How Much Do Water Efficiency Calculations Cost?
Water efficiency calculations are one of the most competitively priced services SES offers. For a single dwelling type, the design stage calculation prices start at £350 plus VAT. Volume schemes with multiple dwelling types are priced accordingly, and when water efficiency calculations are commissioned alongside SAP calculations, we can often offer a combined rate. Contact us for a specific quotation based on your number of plots and dwelling types.
Which Authorities Require Water Efficiency Calculations?
Water efficiency calculations to the 110 litre standard are routinely required across the South East Water, Affinity Water, Thames Water, and Anglian Water supply areas, which broadly cover Essex, Hertfordshire, Kent, Surrey, Sussex, Middlesex, London, Suffolk, Norfolk, Cambridgeshire, and Bedfordshire. They are also increasingly required by planning authorities in other regions of England where local water resource plans identify growing demand as a sustainability risk. If you are unsure whether the optional requirement applies to your development, SES can review your planning decision notice and advise you definitively.
Request Water Efficiency Calculations
Contact SES on 01279873380 to discuss your project. Please provide the planning reference, the number and types of dwellings, and your current fittings schedule if one has been prepared, and we will provide a quotation and confirm our availability to carry out the calculations within your required timescale.
