One helpful way to tackle this assignment would be to break it down into a set of case studies. Since Common Ground gives you a the comparative experience of three different families living through the same moment in history, you have three easy examples to structure your assignment around.
First, think about what you know about each of these families. How are these families different from each other? How are they the same?
Since your prompt specifically deals with questions about architecture, geography, and identity, come up with a set of questions to ask yourself about each family that focuses on these topics. You know the parameters of your assignment better than I do, so you may want to tailor your questions more specifically, but here are some examples that might help you get started:
- Where does this family live, and what is their home like? What's the neighborhood like? What are the advantages and disadvantages of living there?
- How do the individual members of this family engage with each other? How does this family as a whole engage with its community? What drives that connection? Is it racial? Cultural? Based on income bracket or profession? Shared interest? Something else?
- How might the lives of one of these families be different if they all had to switch houses with each other? How, if at all, would they stay the same?
If you answer your questions thoroughly about each family, you'll have a helpful outline that you should have no problem expanding into a comparative essay. Good luck!