I just built a building based on this concept:
Mine has a 12X16 screened room, 8X8 bathroom and 4X8 filter and storage room. It will also have a loft over the bathroom for winter storage and for bulky things like the safety cover during the summer.
We wanted the screened area for dining and shade because we live on a farm and have many flies. Also gives us someplace we can sit at night with lights on and not be overwhelmed with bugs. We poured a concrete floor and used vinyl siding for ours. I'll post pictures when I get caught up with the plumbing and trenching and wiring and fence building and......

so the place doesn't look so much like a war zone.