Is the air space that is occupied by a three dimensional figure. Typically on a registered strata plan the air space contained within the unit or a car space is one cubic space. Internal walls not depicted on a strata plan are typically non common property walls that are outside the ambit of the common property jurisdiction unless they contain assets that service other lots.
VERTICAL AND HORIZONTAL BOUNDARIES:
The boundaries of the common property are denoted by a thick black line. Unless the strata surveyor specifically endorses a note on the strata plan the boundary will be:
1.Vertical boundaries: inner surface of the boundary wall that corresponds with the floor plan ie the thick black lines defining the lot.
2.Horizontal boundaries: which are the upper surface of the concrete slab/original tiles and the under surface of the ceiling.
Normally, almost everything within this cubic space is not common property as it denotes non common assets which are used solely by and for the individual lot.
STRUCTURAL CUBIC SPACE:
Structural cubic space means, cubic space which is occupied by a vertical structural member – i.e. structural columns or a supporting pillar, service voids and any pipes, wires or cables that service more than one lot.
If your asset falls is within the cubic space, and the boundaries of your lot, but outside of structural cubic space, then the asset is NOT common property.
NSW fair trading provides some good information which is useful.