Water
Calculations
Part G compliant — from £10.
Part G Water Efficiency Calculations for new-build dwellings. 125 l/p/day and 105 l/p/day London Plan targets modelled, bundled with SAP at no admin cost, unlimited specification revisions.
- Part G compliant
- London Plan tighter targets handled
- Unlimited specification revisions
£10
Calcs From
125 l/p
Part G Target
105 l/p
London Plan Target
5–10 Days
Typical Turnaround
What is a Water Calculation?
Water Efficiency Calculations predict how much water a new dwelling will consume per person per day, based on the sanitary fittings and appliances specified. They are used to demonstrate compliance with Part G of the Building Regulations, which caps predicted use at 125 litres per person per day.
The calculation is essentially an input-output model: wet rooms and appliances in, per-person daily consumption out. Each fitting has a standard flow rate assumption — or a manufacturer-declared flow rate if you specify a specific model. We compare total modelled consumption against the applicable target and flag the cheapest spec changes if you exceed it.
Why is the target lower in London?
London Plan boroughs typically require 105 l/p/day (rather than the standard 125 or the "optional higher standard" of 110), driven by water-scarcity pressure in the South East. Meeting 105 l/p/day usually requires deliberate spec choices — low-flow taps, dual-flush WCs, efficient shower heads, and sometimes rainwater or greywater recycling.
When is it needed?
You need a Water Calculation for every new-build dwelling and for most conversions into new dwellings. It is usually delivered alongside the SAP Calculation and may also be requested at planning stage — particularly for London Plan schemes and Energy Statement submissions.
Water calcs for every project.
From single-plot self-builds to large residential developments, bundled with SAP or as a standalone piece of work.
New-Build Dwellings
Every new-build dwelling in England and Wales needs a Water Calculation to demonstrate Part G compliance at Building Control handover.
- 125 l/p/day Part G target
- From £10 + VAT per dwelling
- Bundled with SAP as standard
Conversions to Dwellings
Barn conversions, office-to-residential, pub-to-flats and other change-of-use schemes that create new dwellings all typically need a Water Calculation alongside the L1B SAP.
- Change-of-use conversions covered
- Specification advice if existing fittings over target
London Plan Projects
London Plan boroughs require 105 l/p/day — tighter than the standard Part G cap. We model against the correct borough-specific target and flag spec changes needed to pass.
- 105 l/p/day target modelled
- Integrated with Energy Statement
Strict-Target Authorities
Some local authorities outside London adopt the "optional higher standard" of 110 l/p/day, or their own bespoke water-efficiency conditions. We handle both.
- 110 l/p/day higher standard modelled
- Bespoke authority targets handled
Part G sign-off without the faff.
Fast turnaround, London Plan tighter targets handled, bundled with SAP for zero additional admin, and unlimited specification revisions if fittings change.
Clear value up front
Advice, unlimited revisions, SAP bundling all included. No hidden or extra fees at Building Control handover.
Part G Fully Covered
Every Water Calculation we produce demonstrates Part G compliance for Building Control, with fitting-by-fitting flow rate analysis and a clear pass/fail summary.
125 l/p/day Modelling
We model predicted water use against the standard 125 l/p/day target and against the optional higher standard of 110 l/p/day where it applies.
London Boroughs' Tighter Targets
London Plan boroughs typically require 105 l/p/day. We know which boroughs apply which targets and apply the correct benchmark from the start.
Fast 5–10 Day Turnaround
Most calculations turn around inside a week once the fittings specification is complete. Working to a Building Control deadline? Tell us and we will prioritise.
Bundled With SAP At No Admin Cost
When ordered alongside a SAP Calculation, the Water Calculation adds very little to the fee and zero additional paperwork for you — we pull the data from the same source.
Unlimited Specification Revisions
Change the kitchen tap, swap to a dual-flush WC, drop a different shower head — revise the spec as many times as needed at no extra charge.
What we need from you.
Six inputs drive the calculation. The more of these you can share up front, the faster the turnaround and the fewer revisions you'll need.
Floor plans
Plans showing every wet room — kitchen, utility, bathrooms, en-suites, cloakrooms and separate WCs. Wet-room count drives the calculation inputs.
Wet room count
The exact number of kitchens, bathrooms, en-suites, cloakrooms and utilities — each has different sanitary assumptions under Part G.
Fittings specification
WC dual-flush volumes, tap flow rates, shower flow rates, bath capacity, and any internal rainwater harvesting or greywater systems.
Appliance efficiency
Washing machine and dishwasher water consumption per cycle. Manufacturer model number lets us read the flow rate directly from the datasheet.
Rainwater harvesting
If specified, rainwater harvesting contributes to the calculation — we'll include it in the modelling and confirm the Part G credit available.
Greywater recycling
Similarly, greywater systems reducing potable demand are counted in the calculation. Common on tight-target London Plan sites.
Three steps to Part G compliance.
From first call to certificate in your inbox. Fast, bundled-with-SAP convenient, and revisable as many times as needed.
Send Us Your Plans
Floor plans plus fittings specification (WCs, taps, showers, baths, washing machine, dishwasher). We confirm the correct target — 125, 110 or 105 l/p/day — and quote a fixed price.
We Calculate
We model predicted consumption against the applicable target. If the spec exceeds the cap, we recommend the cheapest change — usually a lower-flow tap model or a dual-flush WC.
Report Issued
You get a formal Part G water efficiency report with the consumption figure, target comparison and fitting-by-fitting breakdown — ready for Building Control handover.
Transparent guide pricing.
Send us your plans and fittings spec for an exact fixed price. Cheapest when bundled with a SAP — we pull the data once and produce both documents.
Starting price per dwelling when bundled with a SAP Calculation. Standalone water calcs are quoted individually. Volume discounts apply for multi-plot developments.
- Part G compliant report
- 125 / 110 / 105 l/p/day targets all covered
- Specification advice if over target
- Unlimited revisions as fittings change
*This is a guide price. Standalone calculations without a bundled SAP are priced individually. Contact us for an accurate quote.
Get your Water Calc quote
Takes 60 seconds. No obligation. Bundling with a SAP? Tell us — we'll apply the bundle price and handle everything together.
Water calc questions.
Everything you need to know about Part G Water Efficiency Calculations — targets, inputs, timing and bundling with SAP.
Still Got a Question? Ask Us What is a Water Efficiency Calculation?
A Water Efficiency Calculation predicts how much water a new dwelling will use per person per day, based on the specified sanitary fittings and appliances. It's used to demonstrate compliance with Part G of the Building Regulations, which caps predicted use at 125 litres per person per day (lower in many local authorities).
When do I need one?
For all new-build dwellings, and for most conversions that create new dwellings. It is usually submitted alongside your SAP Calculations. Some local authorities also ask for it at planning stage — particularly in London and the South East, where tighter targets of 105 l/p/day or less often apply.
How much does a Water Calculation cost?
Water Efficiency Calculations start from £10 + VAT per dwelling. They are so inexpensive because they are almost always bundled with a SAP Calculation — if you are ordering a SAP with us, the Water Calculation adds very little to the fee.
How long does it take?
Most Water Calculations are turned around within 5–10 working days of you sending us the information. Because the calculation itself is fast, turnaround is mostly gated by how complete the fittings specification is.
What documents do I need to provide?
A complete set of floor plans showing wet rooms (kitchen, utility, bathroom, en-suite, WC), and the specification for every water-using fixture — WCs, taps, showers, baths, washing machines, dishwashers. Manufacturer model numbers let us look up the flow rates directly.
What is the 125 l/p/day target?
Part G of the Building Regulations caps predicted water use at 125 litres per person per day for new dwellings. The "optional higher standard" drops this to 110 l/p/day and some authorities (particularly London Plan boroughs) require 105 l/p/day. We confirm your specific target at quote stage.
What happens if my specification exceeds the target?
We recommend the cheapest route to compliance — typically a lower-flow tap model, a dual-flush WC, or a more efficient shower head. Swapping one or two fittings is usually enough to drop consumption below the target, without any visible change on site.
Do I need a Water Calculation for an extension?
Not usually — Part G water efficiency applies to new dwellings, not to extending existing ones. If your extension creates a new dwelling (e.g. an annexe with independent sanitary facilities) then yes. Tell us what you are doing and we'll confirm free of charge.
Can I bundle this with my SAP and Energy Statement?
Yes — and we recommend it. One provider delivering SAP, Water Efficiency, Part O (where relevant) and the Energy Statement keeps all the numbers consistent across planning and Building Control documents. Bundling also typically saves admin time and fees.
Do you cover all UK regions?
Yes. Part G applies across England and Wales; Scotland and Northern Ireland have their own (but very similar) water efficiency provisions. We deliver calculations into every UK region.