Something I saw years ago being done in Europe - shipping container apartments. Containers are cheap, strong, and stackable. All you need to build is walkways / stairs to access the front of them.
20 foot containers could be used for the most basic housing providing a living area and bedroom. They can be build around a common courtyard with communal bathrooms, showers, laundry.
This is one being built in Austin using 40 ft containers and each unit has it's own bathroom and kitchen.
