Draw a simple floor plan, take some pictures, inside and out, decide exactly what you want, find a civil or structural engineer. Then find a couple of contractors.

I see your message is old but I am in the process of doing this very thing above my 2 story kitchen. We are also in NY and I have seen all the California ones, too. Our room is large and we are building a new master bedroom over it. It will be under 20,000 when all is said and done. None of the builders that have looked at it had concerns...they all said it is very 'do-able".

Adding floor loads to walls, headers, beams and supporting structure below that were not originally designed to be load bearing can be unwise without professional assistance. It is best to have a structural engineer advise you even when the walls look adequate.

Also, it is often necessary to make the new floor structure as shallow as possible in order to provide adequate upper floor headroom and in such a case it is possible to design floor joists that meet the building code but are uncomfortably bouncy.