Approved Document G (Sanitation & Hot Water): What Homeowners Need to Know

Part G is divided into several sections:

• G1 — Cold water supply: every dwelling must have a wholesome cold water supply from the mains or an approved source.
• G2 — Water efficiency: new dwellings and certain extensions must not exceed a daily water consumption of 125 litres per person (or 110 litres in water-scarce areas).
• G3 — Hot water supply and system efficiency: hot water storage vessels must have adequate temperature controls and safety devices to prevent scalding and Legionella growth.
• G4 — Sanitary conveniences: a dwelling must have at least one WC and wash basin.
• G5 — Bathrooms: a dwelling must have at least one bath or shower.
• G6 — Kitchens: a sink with hot and cold water is required.

Part G3 requires hot water systems to be designed to prevent two conflicting risks:

• Scalding: water at the tap should not exceed 48°C (or 44°C for baths serving vulnerable users such as children or the elderly). This is achieved with a thermostatic blending valve (TMV) or thermostatic shower/bath filler.
• Legionella: the bacteria that cause Legionnaire’s disease multiply in stored water between 20°C and 45°C. Hot water storage cylinders must store water at 60°C or above to kill bacteria, and should be able to reach this temperature within 1 hour.

These requirements are reconciled by storing hot water at 60°C in the cylinder and using a TMV to blend it down to a safe temperature at the point of use.

For new dwellings, Part G2 requires that the calculated water consumption does not exceed 125 litres per person per day (or 110 l/p/d in designated water-stressed areas, which includes most of southern and eastern England).

This is calculated using a standardised method based on the flow rates and volumes of fittings installed (WCs, showers, baths, taps, washing machines). Typical specifications that help meet the target include:

• Dual-flush WCs using no more than 4/2.6 litres per flush
• Showers using no more than 8 litres per minute
• Basin taps using no more than 6 litres per minute

A water efficiency calculation (often part of a SAP assessment) must be submitted to building control before work starts.

Adding a new bathroom or en-suite to an extension or loft conversion triggers Part G requirements. Key practical points:

• The drainage must connect to the soil stack in accordance with Approved Document H.
• A thermostatic shower valve (TMV2 or TMV3) is required to control scald temperature.
• If you are installing a new unvented hot water cylinder, this is notifiable work — it must be carried out by a competent person (G3 registered) or through building control.
• Ventilation to the new bathroom must comply with Approved Document F (extract fan or window).

Unvented hot water systems (which operate under mains pressure without a cold water tank in the loft) are popular in modern homes because they deliver high-pressure hot water to all outlets. However, they are notifiable work under building regulations and must be installed by a G3-registered engineer (such as a member of a Competent Person Scheme) or with building control involvement.

Safety devices are critical: an unvented cylinder must be fitted with a thermostat, a high-temperature cut-out, a temperature and pressure relief (TPR) valve, and an expansion vessel. Failure of these devices can cause a catastrophic failure.