Where the Design Comes From: the Law, the Standards, and Which Drawing Answers Which Question

A site meeting. The architect has the drawings, the electrician has BS 7671, the surveyor has the bill of quantities, and somebody asks who is bringing the Water Regulations Guide. The answer is the plumber — because the duty falls on whoever installs the fitting, not on whoever drew it.

The short answer

Two things are law: the Water Supply (Water Fittings) Regulations 1999, and the Building Regulations. Everything else is recommendation — and following it is the accepted way of showing the law has been met.

Design guidance for cold water systems inside buildings is BS EN 806, in five parts, with BS 8558:2015 as the UK complementary guidance alongside it. Commissioning procedures are in BS EN 806-4, section 6. Those two answers are worth fixing in memory, because the exam offers BS 7671 (wiring) and BS EN 12056 (drainage) alongside them to catch anyone picking a familiar number.

Regulation 3, and the design rule that follows from it

Regulation 3 forbids any fitting installed, connected, arranged or used so that it causes or is likely to cause waste, misuse, undue consumption, contamination or erroneous measurement.

Almost everything in cold water design is Regulation 3 wearing different clothes. Servicing valves deal with waste, because a dripping float valve can be isolated. Avoiding dead legs deals with contamination. Limiting run-off at outlets deals with undue consumption.

Which gives the rule that matters most: lay the system out so those harms cannot arise, and only then add devices such as check valves where the layout alone cannot do the job. Designing the layout first and the devices second is not a style preference — it is what a device-heavy design is admitting it got wrong.

And Regulation 5 notification is part of the design work, not an afterthought for the office. If your design includes a booster over 12 l/min, a bath over 230 litres, an RPZ or a pipe outside the 750 to 1,350 mm depth band, the notice is a design deliverable.

What each standard actually gives you

• BS EN 806-1 — general. Terms and definitions used by all five parts.
• BS EN 806-2 — design. System design; clause 19.1 storage cisterns and recommended storage capacities.
• BS EN 806-3 — pipe sizing. The simplified method: Table 2 draw-off flow rates and loading units, Table 3 pipe sizes per material, pressure and velocity limits.
• BS EN 806-4 — installation, filling, flushing; commissioning and pressure testing in section 6.
• BS EN 806-5 — operation and maintenance. Tasks and intervals.
• BS 8558:2015 — UK guidance alongside: Table 5 design flow rates, Annex A boosted systems, Annex C traditional UK loading units and the tabular sizing method, and the backflow protection tables.
• BS 6700 — withdrawn, but you will still meet its figures in older drawings and exam questions. Use it for method, and say when a figure has been replaced.

Around them sit BS 8000-15 (workmanship), BS 1710 (pipe identification colours), PD 855468 (flushing and disinfection) and HSE ACOP L8 (control of Legionella).

The manufacturer beats the standard

Standards give general figures; the manufacturer gives the figures for the appliance you are actually fitting. BS EN 806-3 Table 2 gives a shower head a flow rate — and also says to check with the manufacturer for non-domestic appliances. BS 8558 Table 5 says the choice of outlet fitting affects the flow delivered, and manufacturer's advice should be obtained.

When commissioning a mechanical component — a pump, a PRV, an accumulator — it is the manufacturer's instructions you consult, not a price list, the client's brief or the risk assessment. Ignoring them voids the warranty and can put you in breach of the Regulations without knowing it.

Two documents both carry the performance requirements: the installation specification says what the system must achieve, and the manufacturers' technical data says what each appliance needs in order to achieve it.

Which plan answers which question

A new estate, thirty plots, and you need to know where each dwelling's boundary stop valve will be. The architect hands over a roll of drawings, and picking the wrong one wastes the morning.

The scale is a ratio. At 1:100, 10 mm on paper is 1 m in the building. At 1:50, 10 mm is 500 mm. At 1:500, 10 mm is 5 m. The smaller the second number, the larger the drawing and the more detail it carries.

Heights come from somewhere else

The floor plan gives you the horizontal run to the bathroom. The pipe also has to climb, and no plan can tell you how far, because a plan looks straight down.

An elevation is a view of one face of the building, so a side elevation gives the height of a storey, an eaves or a cistern platform. A section cuts through and shows floor-to-floor heights and the loft. A schematic or isometric shows the pipework itself with every junction, valve and appliance — usually not to scale, though it is good practice, from the withdrawn BS 6700 (Annex D), for the isometric used for pipe sizing to be drawn to scale, with each junction numbered for the calculation sheet.

To measure off a scale drawing: lay it flat, read the scale from the legend in the title block, pick the matching edge of the scale rule, put the zero on the start of the line and read the length. The commonest error is reading the wrong edge, because most rules carry two scales on each edge.

No drawings? Measure it, and pick the right instrument

An existing 1930s semi, a new bathroom, no drawings in the house. Even on a new build the drawing and the building do not always agree, so measurements are taken in situ, in this order of accuracy:

• Laser measure — the most accurate, about ±3 mm, one person, unaffected by sag or obstacles.
• Measuring wheel — for long external runs; more accurate than pacing, a folding rule or scaling off a plan.
• Tape measure — needs two people, and reads long if it sags or bends round an obstruction.
• Ultrasonic measure — quick, but thrown by obstructions.

Write down every section length as you go, because those lengths become the rows of the calculation sheet, and a length guessed later is a pipe sized wrong.

While you are in the building, survey what the pipes pass through and what the appliances hang on. A wall-hung basin on a timber stud partition needs a timber fixing board; solid brick, block or concrete take fixings directly. Find out which wall before the quotation goes in, not after the basin is in the van.

And look at where the customer wants the cold main to run. Routing it through the airing cupboard beside a hot flow and return is a design fault, not an insulation problem — BS 8558 clause 4.3.5.1.1 says cold water should not be warmed above 25 °C and ideally not above 20 °C, and no thickness of lagging guarantees that in a warm cupboard. Re-route it, then measure the route you will actually use.