A sunken living room, an entry landing, the uphill half of a terraced ground floor: part of one storey sits at a different level from the rest. Split the floor rather than adding a storey, and the two parts stay one floor with one boundary between them. Split levels are free.
There are two ways to cut one, both in the toolbar’s Floors & levels group and both on a selected floor’s panel:
- Split an area — press U, then click the corners of the part that changes level and press Enter. This is the sunken corner.
- Split with a line — click two points across the floor, then the side that changes level. What is stored is the line and the side, not the shape it cuts today, so the split stays put when the house grows. This is the house split in half.
Either way the new part starts one step down from the floor it was cut out of — 300 mm, or 12 inches on an imperial plan — and is selected for you. The panel does the rest.
Zone or storey?
A jump you take in a few steps inside one space is a split. A jump with a real stair between separately drawn wings is another floor — add a storey at the level you want and trace this one under it.
The panel
| Field | What it does |
|---|---|
| Level | Relative to the storey. The note under it reads the finished level against project zero, and says the obvious out loud: changing the level never changes which part owns this area. |
| Thickness | The construction under this part’s finished level. The finished level stays put; the slab grows downward. |
| Finish | The build-up between the structural top and the finished floor. |
| Area | The net area, with what openings and steps take off a gross. |
| Make a separate platform | Turns the part into a platform that stands over the floor and leaves it intact, instead of replacing the floor at its own level. |
| Return to the default floor | Undoes the split and gives the area back to the floor around it. To leave a hole instead, use Create opening here. |
The ±0.00 / −0.60 level tags on the plan read in metres from project
zero, not from this storey. They are derived — there is nothing to keep up to
date — and the tag still selects on a single click, while the floor itself takes
a double-click.
Reshaping the split
An outline split has polygon handles: drag one and both sides update together. A line split has an endpoint handle at each end, plus Start X / Start Y / End X / End Y fields in the panel, and the arrow keys move the whole split. A change that would cross the outline over itself, or make two parts overlap, is refused with a notice rather than half-applied.
The boundary, edge by edge
Each edge between two parts gets its own row in the panel, headed Edge 3 · Down 600 mm, with three choices:
- Closed — a solid edge holds the higher floor back. This is the default.
- Steps — a flight between the two floors. The rest of that edge stays closed.
- Open — nothing is built along the edge and the floor stops in mid-air.
Choose Steps and five more fields appear: Steps occupy (the higher or the lower floor — which side the flight eats into), Position along the edge, Step rise, Going and Width (0 runs the flight the whole way; anything narrower leaves the rest of the edge closed). The rise is a target: the count comes from the real drop, so deepening the split adds a step rather than making the ones you have taller. If a flight cannot fit — a wall covers the edge, or the chosen side is too narrow — the panel says so and leaves the edge closed instead of building treads over nothing.
A floor sitting on made ground has a solid edge whatever you choose. To open it, put a void under the floor in the Site panel, or make the part a separate platform.
What a wall on the boundary does
A wall drawn on the boundary takes its stretch: nothing is generated where the wall already stands, and the wall runs from the lowest slab under its footprint up to its height above the highest one. It is taller on the low side, and the heads of its doors and windows stay in line across the storey — which is what you want, because a wall along the boundary of a sunken room is the normal case.
A door in that wall
A door in a wall that spans two levels gets a Threshold row:
- The high floor — the door is measured from the higher side, so on the low side it opens that far above the floor. This is the default.
- The low floor — the sill sits on the lower floor and the head comes down with it, so the wall above is taller on that side.
- High, with a step down — a flight is generated on the low side, coming down in the risers a split level uses.
The plan, the elevations and the 3D all read the same choice. A door left opening over a drop is reported by the plan check as A door opens over a step.
Furniture, stairs and roofs
Furniture and stairs stand on whichever slab is under them. A roof does not move: its wall plate is measured from the storey’s floor level, so the raked profile is lifted by the same amount as the split.
An older plan
A plan drawn before floors became parts still splits the old way, and its storey panel offers Convert floors…. That reports what would change before anything is written, and the conversion is one undo step. Drawing a Split with a line on such a plan offers the same conversion, because the line split needs the new model to hold on to.
What this doesn’t do
- A split is flat. It is a slab at one level, not a ramp — there are no ramps.
- It is not terrain. For ground that falls across a site, see ground level and surfaces.
- No mid-edge boundary handles. An edge is closed, stepped or open as a whole; the flight’s width and position within it are numbers, not handles.
Next
Floors, platforms and lofts for the floor being split, guarding for the edge above the drop, and stairs if the jump wants a proper flight.