There's no doubt at all that it's much easier to install a wet Underfloor Heating system if it's a New Extension or New Build as there are no existing floors to rip up. That's not to say it's not possible; it'll just take a wee bit more time, money and effort. However, the benefits of underfloor heating will be there long after the pain of installation has worn off.
There is also the option of using a dry/electric Underfloor Heating system. It's certainly easier to install, a bit cheaper, less work to be done underneath, but there can be a difference in running costs.
We can always advise on the best method to go for.
In general existing smaller rooms or existing rooms that are only occasionally used could be natural choices for the dry system as the running and installation costs would balance out.
It would be best if you also considered whether simply adding radiators would solve the problem. We know we are talking ourselves out of a job here, but sometimes it makes sense to go traditional.
Contact us to discuss what's best.
You, too, can be as comfortable as the dog. As long as you don't mind lying on the floor....