Is there any drawback to building a bunch of 1-room buildings? If not, then why would you ever build a multi-building room? Making them separate just gets you more gold.
Because you you have a sensible GM who says "no" in most cases? I for one ask that the players use the example buildings as a baseline when designing their own. I"m pretty sure I remember reading something in the downtime rules that basically instructed the GM to keep players from doing inane things, like building a whole bunch of graveyards.
And you don't profit from a building until the entire thing is complete.
Chemlak, do you remember this? You do get to add your business' bonuses to your own when running it.