I have never heard of anyone burying the trap for a kitchen sink in the floor (depending on your plumbing code it may be illegal because of the distance from the sink to the trap weir). The 6′ to the vent shouldn’t be a problem, but again it depends on your plumbing code and the pipe size. In your situation, if it is a NEW house I would contact the local plumbing inspector. If not new, I would call another plumber and have him check the installation.