A small basin sits on the WC cistern lid and is fed from the cistern's own fill valve: you wash your hands in fresh potable water, and that water drains through a coarse strainer into the cistern below to become the next flush. Because it borrows the supply and the drain the cistern already has, it needs no second water pipe and no waste connection, which is what makes it a genuine retrofit to an existing cistern WC rather than a plumbing job. Saves roughly 1.5 litres of potable water per handwash, on the order of 6.5 cubic metres a year for a household of three, by making one volume of water do two jobs. Worth being precise about the water quality: what reaches your hands is fresh potable water, never recycled; only the already-used water is reused, and only for flushing. Distinct from the separate greywater-diversion entry, which is a building-scale plumbing diversion and provides no handwash point of its own; this is a self-contained appliance whose primary purpose is the handwash point.
Note: Cost, carbon and footprint are rated against the other options for this same need, not in absolute terms.
An existing cistern-type WC whose lid the unit can replace or sit on, and a pressurised cold supply already feeding that cistern
No sizing or quantities: how much a technique yields, and how much a household needs, both depend on the build and the people, so the catalogue compares options rather than dimensioning them. It carries no bill of materials and no build sequence, and it is written for an oceanic, clay-soil climate (Normandy, France). Regulatory notes are indicative and need confirming locally.