Open the floors menu in the top bar — the button naming the storey you are on — and choose Empty floor below, or Duplicate walls below to copy the current floor’s structure down. The new storey goes underneath the bottom floor at a negative level, becomes active, and is named Basement.
What the editor does with it
- The level is negative. It is set to the bottom floor’s level minus the current floor’s wall height and slab thickness, so a basement under a standard ground floor sits roughly a storey down.
- A second one is named after what it goes under — once a floor below zero exists, the next one down is called Below <that floor’s name> rather than a second “Basement”.
- The plan still opens on the ground floor. Floors are stored bottom to top, so adding a basement makes it the first in the list — but reopening the plan puts you back on the storey at ground level, not in the cellar.
- The 3D view digs for it. The ground plane is cut away where the building goes below it, so the basement is visible instead of being buried, and the ground still reads as ground everywhere else.
Tracing the floor above
A basement is drawn against the building above it, which is the one case where the default ghost points the wrong way. In the floors menu, under Trace over, pick the ground floor by name — any floor can ghost any other, in either direction.
Ground level and the plinth
With nothing selected (and any tool but the wall tool), the properties panel shows a Site section with a Plinth field: how high the ground floor stands above the ground outside, in millimetres, up to 3000. That one number is the site datum. It draws the plinth in 3D and the ground line on every elevation, and because it is stored as an absolute level rather than an offset, a basement below it needs no special handling — it simply sits under the ground line where it belongs.
If your basement looks wrong in 3D or on the elevations, this is almost always the field to check first.
What this doesn’t do
- No retaining walls, drainage or waterproofing detail. A basement is a storey like any other: walls, openings, a slab. The below-ground construction is not modelled.
- No sloping site. The ground is one flat plane at the site datum, so a plot that falls across the building cannot be modelled. Individual paved surfaces can be set above or below ground and given a fall, but the ground itself stays level.
- No stepped basement. Like every other floor, it is one flat level.