When should I plan the heating for an extension?
Before the floors go down. It's much easier and cheaper to get it right at that stage.
Why underfloor heating for an extension?
For a new kitchen or living-room extension, I'd generally favour underfloor heating if the budget allows. You've no radiators taking up wall space, the heat is nicely distributed and it's especially comfortable in larger open-plan rooms.
Why radiators for an attic conversion?
I'd usually lean towards radiators. They're easier to install, respond quicker when you turn the heating on and don't involve building up the floor unnecessarily.
Can my boiler handle the extra rooms?
Just because your boiler heats eight radiators doesn't automatically mean it'll comfortably heat another three.
The mistake I'd watch out for is someone adding two or three radiators onto an existing heating system without checking whether it can actually handle them.
I'd calculate the heat loss in the new rooms and check the boiler or heat pump capacity, pipe sizes, circulation and heating controls. If you're mixing underfloor heating with existing radiators, we may also need separate temperature controls and a mixing arrangement.
So which should I choose?
Underfloor heating for a properly insulated new ground-floor extension, and correctly sized radiators for most attic conversions. But I'd always design it around the house, not just what looks best on the drawings.