A guide for architects: how to specify a bathroom

A bathroom specification outlives the moodboard. Between the concept and the site stand a tender, an order, a delivery and an installation — and at each of those stations the specification is read by different people. This guide gathers the rules that make a specification unambiguous: the language of article numbers.

The article number is the unit of the specification. It encodes model, size and finish with no room for interpretation — the same line means the same thing in the architect's office, in the retailer's quote and on site. A specification written in words ('black basin mixer, tall') is an invitation to improvise; a specification in numbers is a contract.

The finish is part of the number. The same model in chrome and in matte black is two different articles — at Hansgrohe, for instance, the finish is encoded in the final digits after the hyphen. So the number is always written out in full, suffix included: a truncated number is the most common way a bathroom arrives in the wrong finish.

Structure the specification by room and by position: each position with its number on the drawing, product, article number, finish, quantity and an installation note. One table per bathroom, one line per item — and the site reads itself without phone calls.

Concealed bodies are specified first — and flagged for early ordering. Mixer rough-in bodies, WC frames and linear drains determine the pipe runs and the heights, and must be physically on site before the plumbing stage. Set them apart in the specification with a clear label: 'order first'. Trims can wait for the tiles; the bodies cannot.

Do not forget the invisible lines: traps, wastes, angle valves, sound-decoupling sets, extension sets for concealed bodies where the build-up is thicker. These are the small article numbers that stall sites — the missing extension set is noticed only once the tile is already fixed.

Heights and axes belong on the drawing, not in prose: flush plate height, mixer axis, shower head height, basin centreline. Numbers on the wall elevations spare interpretation on site — and interpretation on site always works against the design.

Use factory data as the source of truth: manufacturers' product pages provide installation drawings, tender texts and CAD/BIM data for almost every article. A link to the factory sheet at every position makes the specification self-sufficient — everyone down the chain checks the source, not the retelling.

Discipline on substitution: 'or equivalent' is a dangerous clause in bathrooms, because systems do not mix — a flush plate from one system does not fit another's cistern, and a trim from one brand does not sit on another's body. If substitution is allowed, restrict it explicitly to the same system; otherwise a paper equivalent becomes an incompatibility on site.

Finishes are approved in person: the screen lies about both tone and texture. For coloured finishes, request a physical sample before final approval, and keep one family of finishes across all rooms — so the project reads as a single hand.

Finally, working with us: for architects and studios we run a trade programme — project terms, help with the specification and a single point of contact from the first line to the last delivery. Write to us with the project; we speak the language of article numbers.