Press S for Stairs, click to place, then pick the type:
- Dog-legged: the usual Indian stair, two flights side by side with a half landing.
- Straight, Straight + landing, L-shaped, L with winders, Double winder, Open well.
Set Flight width, Turns and Height to climb. The steps and Riser / tread are worked out from the floor height, with a warning if it gets too steep. Press Q to turn it 90°.