[electromagnetic]

Run a forced air line to the lowest level and run regular central air from the lowest level (rather than simply the basement, you'd be running it from the 2nd or 3rd sub-level).

It would also be ideal to ensure the basement is fully sealed (IE the moisture wrap they bury after you've had a leak can actually be laid before the foundation) which will help dramatically with the moisture in the basement levels.

I would imagine a house would become more vertical in narrower confines. I know my friend had a house that was only 12ft wide but long. The ground floor was kitchen and dining room, 2nd floor was a huge living room with a small bathroom, 3rd floor was 2 bedrooms, 4th floor (attic conversion) was the master bedroom with a huge en-suite bath.

I don't understand why you'd build a single story home on a <500sqft lot. A spiral staircase doesn't have a significant impact on square footage and a second or third level would dramatically increase the usable area rather than keep at the limit of ground level.