Open floor
What is there
It starts with one continuous area inside the existing walls. Generous, but with only one function. The dimension shown is the clear span between the existing internal faces.
The plan shows only the existing external walls and one undivided floor area.
New wall
The division
A stud wall splits the area. Where it sits decides how large the two new zones become. Because it carries no load, that line is almost free to choose; it only has to meet the floor and ceiling properly.
A slim new wall now stands in the plan and divides the area into two zones.
Doorway
The access
An opening goes into the wall with reinforced studs and a lintel. The hinge side and swing direction are agreed beforehand, because they decide how the room can later be furnished.
A door opening is drawn into the new wall, with a leaf and its swing arc.
Ceiling
The second plane
A suspended ceiling lays a new plane over the room. Above it is room for services and ventilation, below it a flat surface that can carry recessed lighting.
A dashed grid over the new zone represents the suspended ceiling.
Finished room
The result
Boarded, filled and sanded, the floor area has become a room. Two zones, one doorway, one continuous ceiling plane, all ready for paint and flooring.
Both zones are shown as finished rooms, with floor areas and room references.