To serve anew radiator in a new extension can the feed and return copper pipes be hidden in the concrete floor under tiles?
If so, do I need to take any extra precautions?
Yes but they must be protected from corrosive effects of the concrete, they must have room to expand / contract and there must not be any joints where you can't get at them.
To protect against corrosion you can get a felt lagging from a plumbers merchant, comes as a roll and you push the pipe through. I've also used a tarred strip. They stop the concrete from coming in contact with the copper and allow some expansion
