JohnMo

If can walk on it even if not designed for walk on it needs fall protection. So you need to stop them going on the green roof. Remember you cannot use horizontal bars etc that a child could climb up and over

That can be attenuated with suitable vents that knock the noise transfer down to nothing.

Greenwood CV2GIP, silent, smart humidity stat, easy to install, commission etc. cheap off eBay sometimes available. But to work any fans need somewhere to draw from, so you need something on the other side if the room like a trickle vent always open. But I would list what ventilation is actually installed now and look at the whole house ventilation strategy in the round, before fixing one thing.

I saw them for sale in a garden centre a couple of months ago, they are still sold. It didn't come with the fire place, but was under £50.

But not for an airtight house with the MVHR off or no windows open!

Or the other alternative is a combi storage boiler such as a Viessmann 111-W it has the cylinder built in to give the same effect in a compact package, 25kW boiler will modulate down to 3kW. Will 20l/min DHW flow rate. Install is the same as a standard combi, no complications.

All true, but it keeps the wife happy knowing we could light the fire.

This was mine for ICF build (external insulation), I added a 70mm up-stand to the the screed edge also, not shown on the drawing

Overall less expense than a UVC and boiler combination. No servicing required unike an UVC, pretty simple to install overall. If OP doesn't have space leave it out. But for anyone with a Combi boiler great performance for nor money, and easier cheaper than an UCV

At no-where near that age, I will still have enough wood left from our build, I suspect by the time I get there. So for the amount of time we use it, generally only once in a while and only two logs each burn, I will still keep it. Also I think we have had about 10 power cuts in the last 12 months, one for over 24 hrs, the others a few hours each time. So its nice to have the secondary heating system.

Depends how far is it travelling 1m or 100m or something in-between?

Why hasn't your architect not already planned this out and structural engineer not done the supporting drawings?

Higher flow rate than you would otherwise get from a combi, due to a lower dT required to be added to the warm instead of cold water, especially in winter. Consistent output flow rate winter and summer. In fact our hot water flow hasn't changed since we installed an unvented cylinder for the ASHP compared to the combi and pre heat. Simple additions, big performance improvement. What's not to like.

Another read https://www.heatgeek.com/viessmann-vs-worcester-boilers/

Not sure how else you would do it. Block work and lintel over, pea gravel around pipe... Assume you mean by private drain, you mean waste pipes from your own house and not some other buildings? Another buildings waste would be rerouted around the new foundation not through or under it.

You can just as easily design a standard strip foundation to be just efficient and can be completed by your local ground workers

For radiators use this to get a decent radiator size https://www.heatgeek.com/do-i-need-to-upgrade-my-radiators-for-a-heat-pump/

First whole house and room by room heat loss calculations. From the room calculation you find the size of the radiator required for each room. dT 15 means the flow temp I'm the radiator is 15 degC hotter than room target temperature. This will get you most of the way https://www.heatgeek.com/how-to-size-my-heat-pump-or-boiler-heat-loss-cheat-sheet/

Need to be as close as possible realistically.

Says nothing meaningful that doesn't really apply to most ASHP. They don't really care if it's silent or the most noisy available - the same condition applies.

All I say is, have you seen the price? Not exactly the cheaper ASHP on the market. How many dBA? And at what distance?

Basically you need a powerful boiler BUT it needs to modulate down well for heating so sub 6kW at mine modulation, is ideal but the lower the better. Atag, Viessmann would be my go to boilers. I have used a superflow with A series Atag combi and they are excellent but you need two things to make them work well, remove cold inlet restriction, that is fitted to all UK boilers. Have a boiler that specifically takes pre heated water (most don't) so it can modulate effectively. Have run 3 showers from my Atag combi and superflow setup at the same time, no effect switching on a cold water tap either. What ever you get make sure you don't flood the place with thermostats as all you will do is make the CH ineffective and make the boiler short cycle. If you can run as a single zone on WC. Both the manufacturers mentioned do that out the box.

None at all, if I was to build again I would do a Larson strut build on site. Current house is woodcrete ICF. Masonry can take or leave it. Not a preferred method locally, so trades harder to come by. Not keen on render either - bricks not local either as no clay to make them from. Any build method has good points and bad. Either can be dreadful or passivhaus standards. Kit houses I am sceptical about, after pushing to go that direction on our build - we were sold a high speed build, until I started asking questions and digging deeper. U values we wanted were not achieved, build time not achievable, cost stagging. So changed all our plans to ICF and did it myself for half the cost, better airtightness and good U values.

Think all houses around us are timber kits. Doesn't mean there any good. That's down to the build process and the people doing the work.

Soak the floor and beds with water? The evaporation will draw some heat away. But its a green house what do they expect to occur?