You can include multiple mass instances in a project. To eliminate overlap, use the Join Geometry tool to join a mass instance to other mass instances.
As a result, their gross volume and gross floor area values are adjusted accordingly.

 Geometry panel
Geometry panel Join drop-down
Join drop-down
 (Join Geometry).
 (Join Geometry). The overlapping form of the first mass cuts into the second mass. The mass floors of the second mass are adjusted to report accurately gross floor area in a mass schedule.
If you move the joined mass instances, their properties are updated. If you move the mass instances so that they no longer intersect, a warning message displays. You can unjoin them using the Unjoin Geometry tool.