Check if room is enclosed?

I have a simple building system (walls and floors/ceilings). How can I check if built room is fully enclosed? Including the possibility that there are additional walls in the room etc.

There is a lot you can do to clarify the intent behind your question.

For example, is this 2D or 3D? Are these walls/floors/ceilings comprised of large Meshes/Colliders or are they built from numerous blocks/tiles?

At any rate, following the approach @logicandchaos suggested, if you scan the room by way of squares (2D) or cubes (3D), you could check for contiguous space to designate as a “room” and determine whether or not it’s fully enclosed, essentially on the logical basis of a pathfinding algorithm (so that dividers can be ignored).

Then, when you’re looking for a “room”, just make sure to set an upper limit on (likely) size, otherwise you might start searching into infinity if a “room” isn’t sealed.

if your game based on grid, search

Flood Fill Algorithm