Linda, Top of the morning to you, Underfloor heating could be a problem with a wooden floor but this you know,the main problem would be the wood will srink, normal timber is around 8%-12% moisture content, so your floor will move,. Today, timber is kiln dried which does not help matters. You could lay the floor but dont fix it around the edges about a meter (3ft) in from the walls, this way if and when the floor moves, the joints can then be pulled in, have another chat with your builder, he seems to know what he's doing. He can then come back after a time to correct the joints, when the floor is dry (% wise) you should have no problems. Good luck. John 2000.