I'm looking to rebuild my lean-to conservatory as a 'proper room'. The idea is to also get about 200mm of EWI put on the (mid-terrace no-cavity 1930s brick) building at the same time (which hopefully will improve the insulation significantly). I'm trying to specify the extension in a way that makes it as easy as possible to install a hydronic heat pump system afterwards (but maybe I should just figure out how to get the builders to work with a heating engineer). Current system is a combi boiler, which is upstairs.

I'm hoping if I get underfloor heating in the new extension (about 15m2) I might be able to avoid having a buffer tank in the house, but I'm wondering what else I should be thinking of. i.e. if I should, for example, specify a large-diameter flow and return pipe to the back of the house where an external heat pump unit might go. Or maybe just put in a big conduit allowing access to the stairwell where most of the existing plumbing goes. I'm planning on at least having a 30A socket fitted at the rear to make it easy to provide power to an external unit.