Building Control-ready PDF reports from £25 +VAT (£30 inc VAT) — covering every identical plot on the site.
Runs entirely in your browser. Autosaves as you work.
Got a letter from Building Control?
If Building Control has asked you for a "water efficiency calculation" or "Part G water calculation" for your new build, conversion or self-build — this is that, and you can do it yourself. No qualifications needed, no site visit, no survey: just the spec sheets of the taps, showers, bath and WCs you're fitting. Here's the plain-English explainer if you want the background first.
What it costs elsewhere
Online services and assessors typically charge £25–£65 for a single calculation — and again for every different house type on your site. WaterMonkey is free to use: build your fittings schedule, see PASS/FAIL live, adjust until it's right. You pay £25 +VAT only when you export the report, one fee covers every identical plot, and extra house types are £15 +VAT rather than full price. For anything beyond a single dwelling, it's not close.
The calculation Building Control wants
Every new dwelling must show calculated water consumption of no more than 125 litres per person per day under Part G — or 110 l/p/d where planning imposes the optional standard. WaterMonkey implements the official methodology, The Water Efficiency Calculator for New Dwellings, in full: Table 1 use factors, multiple-fittings averaging, the water softener rule, and rainwater/greywater systems to BS 8515.
How it works
- Enter your fittings — taps, showers, baths, WCs, white goods; pick a preset and tweak, with live PASS/FAIL as you go.
- See where the water goes — a live breakdown plus practical tips to get under target without downgrading the master shower.
- Add dwelling types — one type covers all identical plots; duplicate and adjust for each different spec, with a per-type plot schedule.
- Export when ready — a branded, Building Control-ready PDF: cover page, results table, PASS/FAIL, and a fittings page per dwelling type.
Pricing that rewards real sites
£25 +VAT (£30 inc VAT) for the report including your first dwelling type, then £15 +VAT per additional type — and a type covers every identical plot free. Typical online services charge £25–£65 per calculation, so for a mixed-type development WaterMonkey is usually the cheapest option by a wide margin. Using it is free; you only pay for the PDF.
Who it's for
Developers and builders who need the water calc for Building Control sign-off; architects checking a sanitaryware spec before it's ordered; SAP assessors bundling water calcs with their assessments (shower flow rates now feed SAP 10 too). Rather we did it for you? Our Part G service has you covered.
Frequently asked questions
I'm a self-builder, not an assessor — can I really do this myself?
Yes. The calculation is a paper exercise based on your fittings' specifications — no surveys, no site visits, no special qualifications. If you can find the flow rate on a tap's spec sheet, you can do this. WaterMonkey handles all the official maths and tells you instantly whether you pass.
What do I need before I start?
The spec details of your fittings: tap and shower flow rates (litres/minute), bath capacity, WC flush volumes, and dishwasher/washing machine consumption if specified. All of it is on product spec sheets or retailer listings. Ten minutes of gathering, ten minutes of entering.
What is a Part G water calculation?
The water efficiency calculation required by Building Regulation 36 and Approved Document G for every new dwelling, using the official methodology (The Water Efficiency Calculator for New Dwellings). It must show calculated consumption of no more than 125 litres per person per day — or 110 l/p/d where a planning condition imposes the optional tighter standard.
Is WaterMonkey free to use?
Yes — building your fittings schedule, checking PASS/FAIL against 125, 110 or a custom target, and exploring what gets you under is all free in the browser. You pay only to export the Building Control-ready PDF report: £25 +VAT (£30 inc VAT), which includes your first dwelling type.
Does WaterMonkey work offline — can I install it as an app?
Yes. WaterMonkey installs on your phone or desktop like an app — use the install icon in your browser's address bar, or Add to Home Screen on iOS — and the calculator then works fully offline. Your work autosaves in this browser; you only need a connection when you export the paid PDF.
How does pricing work for multiple house types?
One dwelling type covers every identical plot on the site at no extra cost — a 40-plot site with one spec is still £25 +VAT. Each additional dwelling type (different fittings spec) is £15 +VAT. Most online services charge per calculation, so mixed-type sites cost a fraction of the usual price here.
Does it follow the official methodology?
Yes — the full DCLG Water Efficiency Calculator methodology: Table 1 use factors, Table 2 multiple-fittings averaging, the Table 3 water softener rule, and greywater/rainwater systems to BS 8515 (Tables 4.6, 5.1, 5.2 and 5.5). Results are verified against our in-house Excel masters.
Can I include rainwater or greywater harvesting?
Yes — both, using BS 8515 intermediate calculations or direct entry, with savings correctly capped against demand and protection against double-counting WC demand across the two systems.
Can I re-export after fixing a typo?
Yes — project details (project name, client, reference, plot schedule) can be changed and re-exported free for 24 hours after payment. Changing the actual fittings calculations requires a new export.