October is when the year-end push starts. Plots scheduled for pre-Christmas handover move into second fix, supply chains get squeezed, and substitutions start creeping onto site: a different shower because the specified one is on back order, a larger bath because the client asked, a tap from whichever merchant had stock.

 

Each of those swaps is reasonable on its own. Together, they are how a scheme that passed Part G on paper ends up with a water calculation that no longer describes the building.

What Building Control Actually Needs at Completion

In England, where the water efficiency requirement applies, the person carrying out the work must give building control a notice stating which water target applies to the dwelling (125 or 110 litres per person per day) and the potential consumption of the completed dwelling. For most work, that notice is due no later than five days after the work is completed.

 

The word that matters is completed. The requirement is about the home as built, not the home as designed. A design-stage calculation is where you prove the specification works. The completion notice is where you confirm the finished dwelling still meets it.

 

The full wording sits in Regulation 37 of the Building Regulations 2010, available on legislation.gov.uk, with guidance in Approved Document G on GOV.UK.

The Substitutions That Break a Calculation

Not every swap matters equally. These are the ones that move the numbers most:

 

• Showers. Flow rate is one of the heaviest inputs. A "similar" shower at a higher flow rate can cost you several litres per person per day on its own.
• Baths. Capacity to overflow counts, and a larger bath is a very common client upgrade late in a project.
• WCs. Swapping a dual flush for a single flush, or a lower flush volume for a higher one, adds up fast because toilets are used so often.
• Kitchen and basin taps. Unaerated taps from a general merchant can undo careful specification elsewhere.

 

On a scheme working to the stricter 110 litre target, there is very little slack. One substitution can be the difference between pass and fail.

How to Keep As-Built in Line With Design

1. Lock the fixture schedule. Issue it with product codes, flow rates, flush volumes and bath capacities, not just descriptions.
2. Make substitutions a question, not a decision. Agree that no water fitting gets swapped on site without the calculation being checked first.
3. Keep the data sheets. For every fitting actually installed, keep the manufacturer's figures. If there is no data sheet, you cannot prove the number.
4. Re-run the calculation before handover. Build your final calculation from what is on the wall, not what is in the tender pack, and have it ready well within the five-day window.

Quick Answers

Do I need a new water calculation if I change a fitting? If the change affects flow rates, flush volumes or bath capacity, yes. The notice to building control has to reflect the completed dwelling, so your calculation should be updated to match.

 

When does the water efficiency notice go to building control? For most work in England, no later than five days after the work is completed. For dwellings in higher-risk buildings, it accompanies the completion certificate application.

Certify What You Built

The cheapest fix is always the one made before a fitting is ordered. The second cheapest is catching it before the completion notice goes in.

 

Fittings changed on site? Input your as-built specs and download your Water Efficiency Certificate in 60 seconds for £25.

 

This article is general guidance and not a substitute for advice on your specific project. Always confirm requirements with your building control body.