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How many cores? How is it installed? This is a useful calculator: https://www.tlc-direct.co.uk/Technical/Charts/VoltageDrop.html

Not a plumber Is there air in the system? Does the pump have somewhere to pump from? e.g. are radiator valves / TRVs open?

I was going to say I'd want the control room bigger if it was for recording. My knowledge is more on a recording build, but essentially bass will be your biggest issue, so lots of mass and decouple everything where possible. You will also want to kill reflections etc especially if you're loud, so soft furnishings will help a lot with this, if you're not able to build anything in at construction. Also bass traps in unused corners (wall ceiling junction is a corner too 😉 )

As above, you need mass your best option is probably MLV PB sandwich Is the space primarily for rehearsals or recording?

Pay particular attention to detailing on doors / windows, these will be your biggest problem. Is that a french door to the left of control room? It will probably be more leaky than others. Consider making stud walls double & decoupled Check your room ratios aren't going to cause acoustic issues I suspect you'll want the room pretty "dead" at that sort of size, so incorporate absorbers / bass traps in the design early on A great resource: https://johnlsayersarchive.com/ Keep us updated on your progress

Looks like a warm roof from the exterior pic. Ventilation through the wall if possible to avoid penetrations to the roof

Something like this?

Presumably you want the reatining wall to be stabalised, so I would be helpful as far as possible without it negatively impacting you significantly. What were the nature of the works you undertook to dig out? Was anyone involved to assess the impact on the wall? It is good that you have photos of the wall before works were undertaken

Just on your diagram - I'd keep services in the space between the plasterboard and the insulation if at all possible, just make sure your cables are run in the correct zones. One tip, if you have at least one socket on each wall, you can run the cable horizontally around whole room, making it easier to add extra sockets in the future. 🍻

Either insulation fixings or plasterboard adhesive foam 🤷‍♂️ - make sure you tape over any fixings Work Go for as much as you can afford / space allows Good question - I'd want a mechanical fixing in there, maybe foil tape over? I'd do some research on best approach. The wall I have done this on has a stud wall in front of it, so no penetrations as fixed to ceiling and floor Work out the biggest depth you need, might be easier with fast fix boxes, but I'm never as happy with them as a proper backbox

Ventilation comment was for the room, not for the walls I was proposing a single continuous layer of PIR, then battens. So your build up from outside in would be: Existing block wall PIR, gaps and joins foamed and taped Battens Plasterboard

Continous layer of PIR with battens at 600mm in front of it giving you a service void? Need to consider ventilation, reveals need some thought too

Loftzone do a product that looks more stable than the cheap legs: https://www.loftzone.co.uk/shop-storefloor/storefloor/?attribute_kit-size=Starter+Kit+1.2m+x+2.4m Another member did an install here:

I'm just shy of 42 degrees, and if I wasn't constrained with it being a renovation and ceiling clearances etc I would have gone shallower. That said it doesn't feel wrong, just not as perfect as you would want in dream build

The outdoor units will have specs of how much space required around them. I don't think N, S, E, W will have a big impact so go with wherever it will be least annoying athestically and acousticly (e.g. don't put next to bedroom window), and factor in pipe runs. Are you having an unvented cylinder for hot water? I would go with a plant room (large cupboard) to house UVC, UFH manifold and controls