Hum, I doubt it would be more to have a basement than extra square footage on the main floor. If you do a basement for bedrooms, I would be sure you have the property grade to allow for windows and a walkout.

I agree about not overbuilding with the forethought of kids leaving home someday. Time goes quickly!

You might want to look at a story and a half. Many of these are built with a separate heating/ac system, so you can close off upstairs when kids leave home, but still have the space when they come back to visit. The footprint would be smaller and you could still have a basement. I don't think stomping should be an issue in either two story or story and a half if built /insulated well.

The cost of the basement vs. the cost of additional main floor square footage will vary a lot depending on your location.

In Utah, basements are common -- in fact, not having a basement will hurt resale. It's rare to find homes with more than 2500 square feet on the main floor (unless you look at mansions).

I don't like stairs, at all, and wanted to build 2700-2900 square feet on the main floor and everyone thought I was crazy and advised me to just move some of the rooms to the basement. The cost per square foot to build and finish basement space is half of what the same space would cost on the main floor.

That being said, I am still trying to get all the rooms that I want on our main level. (4 bedrooms and a den -- in addition to the great room, kitchen, dining, and laundry.)
We've cut back and are now trying to fit in 2400 sq ft.

You could consider putting some rooms over the garage. The kids could make lots of noise up there and it wouldn't bother you in the main living spaces.