Modular design, or "modularity in design", is an approach (design theory and practice) that subdivides a system into smaller parts called modules or skids, that can be independently created and then used in different systems.
http://en.m.wikipedia.org/wiki/Modular_design
Modular buildings can be added to or reduced in size by adding or removing certain components. This can be done without altering larger portions of the building. Modular buildings can also undergo changes in functionality using the same process of adding or removing components.