Trades master copy
Last reviewed: March 2026
What Part G is
Part G is the bit of Building Regulations that deals with three things you hit on nearly every plumbing or bathroom job:
- Clean water in (cold and hot).
- Safe hot water systems (so they don't scald or explode).
- Not wasting water in new dwellings (water-efficiency targets).
The current Approved Document G sets requirements for:
- Cold water supply (wholesome/suitable water).
- Hot water supply and systems (including unvented).
- Sanitary conveniences and washing facilities.
- Bathrooms.
- Kitchens/food prep areas.
- Water efficiency in new homes.
This guide is a summary to make Part G easier to use on site. It does NOT replace Approved Document G: Sanitation, hot water safety and water efficiency (current edition).
You must read and follow the full Approved Document G plus the manufacturer instructions for any unvented cylinders, TMVs and appliances you install.
This guide is written for England. Wales, Scotland and Northern Ireland have their own versions of building regulations - the principles are similar but the documents and approval routes differ, so check local requirements if you're working outside England.
Where it applies on your jobs
Part G comes up on:
- New-build houses/flats - full cold/hot water systems, all WCs, basins, baths/showers, sinks, plus a water-efficiency calculation per dwelling.
- New bathrooms/en-suites/cloakrooms - layout, number and type of fittings, and safe hot water delivery.
- Cylinder swaps and system upgrades - especially unvented hot water cylinders and thermal stores.
- Kitchen refits - sinks and water supply to dishwashers/washers, plus water-efficiency impact if it's part of a new dwelling.
- High-spec or multi-bathroom refurbs - where big baths and powerful showers can easily blow the water-efficiency numbers on new dwellings.
Key "trigger points" - what matters on site
Cold water and basic facilities
Every dwelling must have a suitable installation providing:
- Wholesome (drinking) water at points of use.
- Wholesome or softened wholesome water to washbasins, baths, showers and bidets in bathrooms/WCs.
- Water of suitable quality to WCs with flushing devices.
- At least one bathroom with a basin and a fixed bath or shower, and a sink in any food-prep kitchen.
Hot water safety and unvented cylinders
Any hot water system with storage must be designed to cope safely with temperature and pressure changes in normal use and under fault conditions.
Systems must:
- Prevent stored water exceeding 100°C.
- Use temperature/pressure relief and expansion arrangements sized correctly.
- Convey any discharge from safety devices to a point where it's visible and won't injure anyone.
Unvented cylinders have specific safety kit requirements (TPR valves, expansion vessel, tundish, discharge routes) that must be installed as per manufacturer instructions and Part G.
Scald protection (especially baths)
- Hot water delivered to baths in new homes (including those formed by change of use) must be limited to a maximum of 48°C at the outlet.
- In practice you fit a thermostatic mixing valve (TMV) or similar device that fails safe above 48°C and can't easily be overridden by the user, installed where it can be maintained.
- It's good practice to extend TMVs to showers and other outlets in dwellings with vulnerable users, even where not strictly mandated.
Water efficiency
- New dwellings must meet a maximum daily water-use figure, normally 125 litres per person per day under Building Regs.
- Many local policies cut this further, commonly 110 l/p/d, and some schemes target 105 l/p/d or better - planning conditions will tell you which applies.
- Compliance is shown via a Part G water-efficiency calculation covering WCs, taps, showers, baths and water-using appliances (dishwashers, washing machines etc.).
Quick reference table - common jobs
New family bathroom in a house
- Must include at least a basin and a bath or shower.
- Hot water to the bath must be limited to max 48°C at the outlet - fit and set a TMV accordingly.
- If the cylinder or thermal store is near the bathroom, plan safe discharge routes for any safety valves.
New unvented hot water cylinder
- Only install if you hold the proper unvented qualification; follow manufacturer and Part G requirements.
- Provide at least two independent safety measures to prevent stored water going over 100°C and to discharge safely (TPR valves, expansion vessel, tundish and correctly sized discharge pipework).
- Confirm outlet temperatures at baths are controlled (via TMVs) to 48°C max.
New-build house with multiple bathrooms and "rainfall" showers
- Whole-dwelling water-efficiency calc must still hit the target (125/110/105 l/p/d as applicable) - big showers and deep baths need balancing with low-flush WCs, efficient taps and appliances.
- Sanitary layout must meet minimum numbers of WCs, basins, and baths/showers, with safe hot water at all outlets.
Kitchen refit with new sink and dishwasher
- Provide a sink in the food-prep area with wholesome water to the taps.
- On new dwellings, tap and appliance flow/consumption feed into the water-efficiency calc - don't swap spec to thirsty kit without checking.
Adding cloakroom WC under stairs
- Must have a WC and a basin with wholesome or softened wholesome water.
- Design drainage and venting properly, avoid awkward/under-sized pipe runs.
- If part of a bigger new-build/major refurb, extra fittings add to the water-efficiency total - the spec may need balancing elsewhere.
Routes to compliance for trades
Most of this boils down to: safe hot water design, correct installation of unvented systems, scald protection, and getting the water-efficiency calc right in new homes.
Follow Approved Document G sections G1-G6
- G1 - Cold water supply.
- G2 - Water efficiency (new dwellings).
- G3 - Hot water supply and systems.
- G4 - Sanitary conveniences and washing facilities.
- G5 - Bathrooms.
- G6 - Kitchens and food preparation areas.
Install hot water systems (especially unvented) strictly to Part G and manufacturer guidance
- Don't improvise on safety valves, discharge routes, expansion vessels or controls.
- Ensure TMVs are fitted where required, set correctly and accessible for annual servicing.
Use a proper Part G water-efficiency calculation on new dwellings
- Coordinate early with whoever is doing the calc - they'll set WC/tap/shower/appliance specs to hit 125 or 110 l/p/d.
- If you or the client change sanitaryware spec (e.g. mega-bath, multiple rainfall heads, different appliances), get the water-efficiency calc re-run before you order.
Keep documentation
- Commissioning records for unvented cylinders and any hot-water safety devices.
- TMV locations and temperature settings.
- Water-efficiency calculator output (or equivalent report) for new dwellings.
Who is responsible for what
On a typical domestic project:
- The designer/engineer is responsible for sizing and specifying the water system (vented/unvented, cylinder size, safety devices) and sanitaryware/flow rates to meet Part G and water-efficiency targets.
- The plumber/heating engineer is responsible for installing cylinders, pipework, valves, TMVs and sanitaryware exactly to manufacturer instructions and Part G - especially on hot water safety and scald protection.
- Anyone doing unvented work must have the right ticket and follow the rules exactly - Building Control will expect to see that.
- The main builder is responsible for not boxing in, hiding or compromising safety discharges, or downgrading sanitaryware/water-saving kit without checking the calcs.
- The client/owner ends up with the risk if it's not safe or not signed off.
Blunt version for smaller firms:
If you fit an unvented cylinder or mess with hot water safety devices without doing it by the book, that's your neck. Same if you keep saying "it'll be fine" while the client picks huge baths and power showers and nobody checks the Part G water-efficiency target.
Simple rule to drum into your team
If you're installing or altering a cylinder, a bath/shower, or adding a lot of new sanitaryware in a new build, treat it as a Part G job. Check safety devices, scald protection and the water-efficiency numbers before you close up boxing or sign it off.
On-site checklist (Part G)
Before you start
- Confirm the hot water system type (vented/unvented/thermal store) and identify all safety devices and discharge routes.
- Get the sanitaryware and appliance spec and the water-efficiency target for any new dwellings (125/110/105 l/p/d or as stated).
- Mark which baths/outlets need TMVs for scald protection.
While you're working
- Install cylinders, valves, expansion vessels and discharge pipework exactly as per manufacturer diagrams and AD G.
- Fit and set TMVs correctly, keep them accessible for maintenance.
- Keep pipe runs sensible (no silly dead legs, no vulnerable joints).
When you finish
- Commission and test hot water systems, safety devices and TMVs; record temperatures and settings.
- Confirm each dwelling has at least one bathroom (basin + bath or shower) and a kitchen sink with wholesome water as required.
- For new builds, make sure any changes to fittings/appliances have been fed back into the Part G water-efficiency calc and that the result still passes.
Sources
Based on:
- Approved Document G: Sanitation, hot water safety and water efficiency (current edition on gov.uk).
- LABC and Planning Portal summaries of Part G requirements and sections G1-G6.
- Industry guidance on hot water safety, unvented systems, TMVs and Part G water-efficiency calculations (125/110 l/p/d targets and calculation method).
This guide was last reviewed March 2026. SiteKiln does not provide legal, financial or tax advice. All content is for general information purposes only. Always seek professional advice for your specific situation.
Know someone who needs this?
Working in Wales? The building rules are different. See our Working in Wales guides.
Working in Scotland? Building standards work differently. See our Working in Scotland guides.
Working in Northern Ireland? The system uses Technical Booklets. See our Working in Northern Ireland guides.
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