The family had three bedrooms, four people, and one bathroom. The case for a second bathroom was a domestic-peace case — and it paid back, by their report, within a fortnight.
The under-stair storage cavity was the only candidate space. We ran a structural engineer over the lintel work, dropped a Velux skylight in for ventilation and natural light (no exterior wall available for a window or fan run to the outside), and specified a compact European-style WC and a 900mm corner shower.
Salt-air conditions in Pāpāmoa mean we always specify Methven or Felton tapware over budget brands — they hold up. We did the same here.