I recently came across a post about MorHaus. MorHaus is the brain-child of an architect based out of Charlotte, North Carolina. They are pre-fab modular structures that can be easily assembled in a single day with a 2-person crew.
The sizes of their units range in size, from 8’x12′ to 12’x16′ with the option of joining units with a covered porch between if you need more space.
I have included a few pictures, but here is some additional places you can look at MorHaus.