I think the placement of air returns depends on the individual HVAC contractor, and what they are used to doing if something else is not specified by the owner or architect. The house my cousins lived in (that I mentioned above) was custom designed and built, so every detail was specified as to how it was to be done. I know the place was very expensive when they built it in the mid 80's - over $500,000. I wouldn't expect houses built for the market to include anything that would add to the cost.