So let's just say, hypothetically speaking, that the g.f. wanted to add
some square footage to her house. What are the considerations involved
in choosing between adding a second story, and cutting down on the
amount of lawn she needs to mow?

One factor we had disagreed about is the heating and cooling costs.
Seems to me that for a given total square footage, two story is better,
but she says she's heard otherwise.

I'm guessing there's no hard and fast rules, and surely personal
preference and lifestyle play a significant role, but what about
construction costs, maintenance issues, etc.?