A good friend did something similar. He built a shop something in the range of 30'x30' for him, his wife, and 3 girls (ages about 5, 10, 12 at the time!). He built a bathroom in one back corner and a kitchen area in front of that. He did not build any interior walls. He simply used furniture and portable partiions to section off the rest of the building. They lived in it for a little over a year. Not a lot of privacy, but nothing to tear down when his house was finished. The bathroom is a nice convenience in the shop and the kitchen cabinets became his workbench with a place to wash his hands in the sink.
I built a 30'x40' with the same intentions, although my children are already grown and gone. Here is mine. We plan to build in a couple years or so. I plan to make some temporary portable partitions (basically some simple frames with something like curtains) to separate the different living areas. This was shortly after I built it and moved in some furniture to spend some weekends there. Now after 5 years, you can hardly move around in it without moving some things out first.