Building Regulations Approved Document G Explained

Approved Document G is the part of the Building Regulations that covers cold water, water efficiency and hot water safety in England and Wales. It's short — three functional requirements — but it's the one most domestic plumbing jobs touch. G1 requires every dwelling to have a wholesome cold supply, G2 sets a 125 litres per person per day water-use target for new dwellings, and G3 covers hot water safety, including the cap of 48°C on water delivered to a bath and all the rules for unvented cylinders.

Part G sits alongside the Water Supply (Water Fittings) Regulations 1999 — most jobs have to comply with both, but the enforcers are different. For the bigger picture of how the frameworks interact, read the UK water regulations pillar guide.

G1 — Cold water supply

G1 is the simplest of the three. Every dwelling must have a supply of wholesome water sufficient for drinking, cooking, food preparation and personal hygiene, delivered to a point inside the building. Wholesome means water meeting the Private Water Supplies Regulations or supplied by a water undertaker.

• A wholesome cold tap — typically the kitchen drinking tap — must be supplied direct from the main where possible.
• Non-wholesome supplies (rainwater harvesting, greywater reuse) are permitted for WCs and irrigation only, and must be clearly marked to avoid cross-connection.
• Any water for personal hygiene must be wholesome at the point of delivery.

G2 — Water efficiency

G2 applies to new dwellings. It requires the design to use no more than 125 litres of wholesome water per person per day on average, calculated using the water efficiency requirements in Approved Document G. Local planning policy can tighten that to 110 L/p/d in water-stressed areas — much of the south-east, including parts of Hertfordshire and Essex, sits inside that tighter band.

G2 doesn't apply retrospectively — your existing bathroom is fine — but a refurbishment that changes the property's water-use design (full replumb, extension, change of use) can trigger compliance.

G3 — Hot water supply and systems

G3 is the longest and most technical section of Part G. It applies to all hot water systems — vented, unvented, and instantaneous (electric) — and covers four big areas.

G3(2) — unvented cylinder safety

The unvented cylinder rules are the meatiest part of G3. Three independent layers of protection (control thermostat, high-limit cut-out, T&PRV), expansion provision (external vessel or internal air bubble), a tundish for visible air-break warning, and a properly sized discharge pipe routed to a safe outside termination. Installation must be by a competent person and is notifiable to Building Control. Full breakdown in G3 and unvented hot water cylinders.

G3(3) — discharge pipework

The D1/D2 discharge pipe must be sized per the G3 tables (based on T&PRV outlet size, total length and number of bends), fall continuously from the tundish, and terminate visibly to a safe location. It must never simply spill into a basin, bath or where it could scald a person.

G3(4) — the 48°C bath cap

The headline rule for most homeowners. Hot water delivered to a bath cannot exceed 48°C. In practice this is achieved with a TMV on the bath supply, set somewhere in the 38-46°C range. The rule applies to new builds, replacement baths and material change of use; existing baths in existing homes are unaffected until they're replaced. See safe hot water temperatures for the full context.

What Part G applies to (and what it doesn't)

• Applies to — new builds, extensions, material change of use, and any work that materially affects sanitation, hot water provision or efficiency in a dwelling or non-domestic building.
• Doesn't apply retrospectively — you don't have to retrofit a TMV to every existing bath in the country. But if you replace the bath, the new install must comply.
• Different in Scotland — Scottish Technical Standards Section 3 (environment) does the same job north of the border, with the same broad approach.
• Different in Northern Ireland — NI Building Regulations Part P covers sanitary appliances and hot water.

How compliance gets signed off

1. 1

   Competent person scheme (most domestic G3)

   An installer registered for unvented hot water can self-certify the work. The scheme notifies Building Control and a compliance certificate is issued, typically within 30 days.

2. 2

   Building Control direct

   For work outside any scheme — a self-builder, or a non-scheme plumber — Building Control are notified in advance and inspect during the build.

3. 3

   Water efficiency calculation for new dwellings

   Submitted as part of the Building Regs application; the developer must demonstrate ≤125 L/p/day (or 110 L/p/day where local plans require it).

Frequently Asked Questions

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