Reference: housing built half way for the residents to expand as they like_source MoMA New York

by Elemental, great architects from Chile.
take a look at the video posted at MoMA website: http://www.moma.org/interactives/exhibitions/2010/smallscalebigchange/projects/quinta_monroy_housing
and especially notice the simplicity of diagrams.
units before expansion:

and after expansion: