A lot of good stuff from everybody there. Ladybirder's posts underline an important point. UFH is very different in the way it works. The temperature setting is much lower than conventional rads.
It is important to fire it up well before it's needed. Warm-up time with rads is quick. UFH takes a lot longer.
For running costs - any extra time that it's on is offset by the lower temp. needed, and the fact that it's all thermostatically controlled. With any system, it's all down to "power in - heat out". I doubt if the running costs differ greatly from a rad system. The great advantage is "comfort level". Hard to define, but mainly, it's the fact that heat is spread evenly around the zone, rather than localised at rads. Since heat rises, it eliminates that cold draught around your feet.
New extension? Even easier. Fitting it in the rest of the house would mean raising floor levels significantly.
Have the oversite taken out to a much lower level to allow for a thickness of insulation under the pipework. The oversite is the bit where the floor sits (hardcore/concrete/insulation etc).
I would allow for 100mm Celotex/Kingspan insulation under the pipework... with a conventional sand & cement screed on top ........ or a poured, sloppy one.
Your builders will know all this anyway, but I always think it's nice to know what's going on.