Ondol is one of the things I miss about South Korea. I never had to worry about putting my bare feet on a cold floor if I had to go to the bathroom in the middle of the night!
You had your own bathroom? Man, you guys must be rich! The one room apartment I once lived in off post (Tong Du Chon) had a community bathroom, outside, with no heat.
They put radiant floor heating in the kitchen. Plastic hose was embedded in cement or floor leveler and was part of the forced hot water system. The hose was on the order of a 1/4" or so.
Been a very long while, but they may have then put down slate flooring.