Part G · Design Stage

Water Usage Calculations
Compliance with every drop.

Part G water efficiency calculations for new residential developments, demonstrating that your design does not exceed 125 litres per person per day. Required for Building Control sign-off and increasingly a planning condition.

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What are water usage calculations?

Demonstrating your development's water efficiency under Part G.

Water usage calculations predict the daily water consumption of a new dwelling based on the specification of its sanitary fittings — taps, showers, baths, toilets, and washing machines. Part G of the Building Regulations requires all new dwellings to demonstrate that their design water consumption does not exceed 125 litres per person per day, covering both wholesome (drinking quality) and non-wholesome water use.

As sustainability becomes a national priority and the UK population continues to grow, water efficiency in homes is gaining increasing significance. Many local authorities — particularly in London, the South East, and water-stressed regions — now enforce tighter limits, requiring consumption of no more than 110 litres per person per day as a planning condition.

Water calculations are typically prepared alongside your SAP assessment during the design stage. The result is a direct input into the SAP calculation and affects your overall compliance outcome. Getting the specification right at design stage avoids costly changes to sanitaryware later in the build programme.

Part G water usage limits

125

litres per person per day — Part G maximum, England & Wales

110 L/p/d in water-stressed areas. Many local authorities in London, the South East, and other water-stressed regions set a tighter optional requirement of 110 litres per person per day as a planning condition. We confirm which limit applies to your site.

What counts towards the total. The calculation includes WCs, baths, showers, basin taps, kitchen taps, dishwashers, and washing machines. External water use (garden taps) and non-domestic uses are excluded.

The calculation process

How we calculate your water usage.

We assess every sanitary fitting in your design, calculate the predicted daily consumption, and confirm Part G compliance — typically delivered alongside your SAP assessment.

Fitting schedule review

We review your bathroom and kitchen layouts, identifying every WC, bath, shower, basin tap, kitchen tap, dishwasher, and washing machine. Each fitting's flow rate or volume is recorded.

Water consumption modelling

We calculate the predicted daily water consumption per person using the methodology in the Water Efficiency Calculator for New Dwellings. The model accounts for frequency of use, duration, and flow rates for each fitting type.

Compliance check

We compare the predicted consumption against the applicable limit — either the standard 125 L/p/d or the tighter 110 L/p/d if your local authority requires it. If the design exceeds the target, we advise on specification changes.

Report & SAP integration

We deliver the Part G compliance report and feed the result directly into your SAP calculation. If we're handling both, this happens seamlessly with no delays or additional cost.

What we need from you

Information required for your water calculation.

Water calculations depend on knowing the specification of every sanitary fitting in the dwelling. The more detail you can provide at design stage, the more accurate the result — and the less risk of needing to change fittings later in the build.

Where specific products haven't been selected yet, we can calculate using typical flow rates for the type of fitting proposed. However, specifying low-flow fittings early gives you the best chance of meeting tighter local authority targets without compromising the design.

If your development includes rainwater harvesting or greywater recycling systems, these can significantly reduce the calculated consumption. We include these in the assessment where proposed.

Information required

WC specification Flush volume (single or dual flush) and cistern capacity for each WC in the dwelling.

Bath specification Bath volume to the overflow in litres. Standard baths are typically 150–180 litres.

Shower specification Flow rate in litres per minute for each shower. Low-flow heads can make a significant difference.

Basin & kitchen taps Flow rate in litres per minute. Aerated taps reduce consumption without affecting usability.

White goods Water consumption per cycle for dishwashers and washing machines, if specified.

Rainwater / greywater systems Details of any rainwater harvesting or greywater recycling proposed.

Reducing water consumption

Practical strategies to meet tighter water targets.

If your local authority requires 110 L/p/d or you want to future-proof your design, these measures can reduce consumption without affecting the homeowner's experience.

Low-flow fittings

Specify efficient taps and showers

Aerated basin taps (5 L/min instead of 8), eco showerheads (8 L/min instead of 12), and dual-flush WCs (4/2.6L instead of 6/4L) can reduce consumption by 20–30% with no noticeable difference to the user.

Smaller baths

Right-size your bath specification

Bath volume is often the single biggest factor in the calculation. Specifying a bath with a volume to overflow of 150 litres instead of 180 litres can save 10+ litres per person per day.

Water recycling

Consider rainwater harvesting or greywater systems

Rainwater collection for WC flushing and garden use, or greywater recycling from showers and basins, can dramatically reduce the calculated wholesome water consumption — particularly useful on sites with the 110 L/p/d target.

Why choose us

Integrated with your SAP calculation

Water usage results feed directly into your SAP assessment. When we handle both, there's no duplication, no inconsistency, and no delay between the two.

We know your local authority's water target

Different councils apply different limits. We confirm whether the standard 125 L/p/d or the tighter 110 L/p/d applies to your site before we start the calculation.

Specification advice included

If your design exceeds the target, we don't just tell you it fails. We recommend specific fitting changes — with product suggestions where helpful — that bring the consumption within limit at the lowest cost.

Fast turnaround — 3–5 working days

Most water calculations are delivered within 3–5 working days, typically alongside the SAP report. Urgent turnarounds available on request.

Water calculations near you

Local assessors for Part G water efficiency across the Midlands and South West.

Water calculation requirements and local authority limits vary by region. Our assessors know which target applies in each area and prepare calculations accordingly.

Birmingham & West Midlands

Part G water usage calculations for new builds and conversions across Birmingham, Solihull, Coventry, Wolverhampton, and the wider West Midlands. We confirm the applicable water target for your site.

Bristol & South West

Water efficiency calculations across Bristol, Bath, South Gloucestershire, and the surrounding South West. Some authorities in the region apply the tighter 110 L/p/d optional requirement.

Worcestershire

Based in Worcester — water calculations across Malvern, Droitwich, Evesham, Bromsgrove, and Redditch. Our home county with the fastest turnaround.

Gloucestershire

Part G compliance for residential developments across Cheltenham, Gloucester, Stroud, Tewkesbury, and the Cotswolds. Integrated with SAP and delivered alongside your energy assessment.

Oxfordshire

Water usage calculations across Oxford, Banbury, Bicester, Witney, and the wider Oxfordshire area. We navigate local water stress designations and applicable targets.

Wider England & Wales

Our Part G calculations are accepted by all building control authorities across England and Wales. Contact us wherever your project is.

Looking for water usage calculations near you? Whether you're building a single dwelling in Worcestershire or a multi-plot development in Birmingham — we deliver Part G compliant results integrated with your SAP assessment. Call 01386 365145 or email us to get started.

FAQ

Common questions about water usage calculations.

A Part G water calculation predicts the daily water consumption of a new dwelling based on the specification of its sanitary fittings. It demonstrates compliance with the Building Regulations requirement that water consumption does not exceed 125 litres per person per day (or 110 L/p/d where the local authority has adopted the tighter optional requirement).

Water calculations are required for all new residential dwellings in England and Wales as part of Building Regulations compliance. They are typically prepared during the design stage alongside your SAP calculation. Some local authorities also require them as a planning condition, particularly in water-stressed areas.

125 litres per person per day is the standard Part G requirement that applies to all new dwellings. 110 litres per person per day is the tighter optional requirement that local authorities can adopt through their local plan — this is common in London, the South East, and other water-stressed regions. We confirm which limit applies to your site before calculating.

Baths typically have the biggest impact — a large bath can account for 30+ litres per person per day. Showers are the second biggest factor, followed by WCs. Specifying a smaller bath, a low-flow showerhead, and a dual-flush WC can reduce total consumption by 20–30% without affecting the homeowner's experience.

Yes, but any changes to sanitary fittings that affect flow rates or volumes will require the water calculation to be updated. If the new fittings increase consumption above the target, alternative specifications will be needed. This is why we recommend confirming fitting specifications at design stage.

The water usage result is a required input into the SAP calculation for new dwellings. The predicted hot water demand affects the SAP energy rating — so more efficient fittings not only satisfy Part G but can also improve your SAP score and EPC rating. When we handle both assessments, the results are fully coordinated.

Water Usage CalculationsCompliance with every drop.