ETA: looks like number of days "unqualified use" divided by number of days you owned it times the gain equals the taxable amount. Unqualified use is a really interesting thing, especially if you live half the year in one residence and half in another. Snowbirds will presumably await interpretation.
I think I'm a little bummed about this, primarily because if we rent this house for some period of time before selling it, we'll have to go back and figure out the basis, something we could otherwise have avoided doing entirely. *sigh*
And all the fun of figuring out what can be added back in: the garage, the driveway, the shed, the playset (if we leave it), landscaping, AC, generator, whole house humidifier (assuming we ever get that done). . .