Another reason that was key in our decision to forgo the basement had to do with marketability and appraisal of the final product. At first glance we thought that for marketability a basement would be necessary, but looking at it closely it proved otherwise.
Most of the homes in the neighborhood are about 1,500 to 2,000 square feet total, with half of that footage being in the basement. We will be building approximately 2,000 square feet on two levels above ground, so the quality of our 2,000 square feet should be much nicer than neighbors. But if we were to try to have a basement we would end up with a home that is closer to 3,000 square feet. Even if you figure that you can get half as much value out of the basement as you can upstairs, that would put us in a house that costs 30% to 50% more than neighboring houses, which is definitely a tough sell on an appraisal and could prove difficult in the eventual resale of the house when we are through living in it.
This was verified by the bank when we were getting our financing. They told us that they just couldn't get appraised value out of the basement and it would make the whole financing package that much less desirable to the bank.
So, yes, a basement would make good room for future expansion of the house, but expanding the house to that size would make the house too big for the neighborhood. Plus, like Kersten said, we should see significant savings in construction costs by avoiding the costs for trusses on the first floor, subflooring and finish flooring. Our slab will be all of those things. A slab will also give us really good thermal mass and greater efficiency for our radiant heat system, and perhaps most importantly, a slab will allow us to fit under the 20 foot height required for flat roofs by zoning and still have good ceiling heights.