There are tutorials demonstrating how to incorporate a split level, normally the result of an extension having different floor / ceiling heights. However you may just want to raise a section of floor in a building / room that otherwise has same floor ceiling height.
You can achieve this by using the Room Boundary to define a floor section. Then in the new room created with the room boundary activate the Floor properties dialog (right click and select Properties). In the tree locate Floor-General - Layer construction.
In the Edit selected layer section, you can then raise the floor level using the Thickness field.
The floor level will then raise by the value entered.
