JohnMo

I would do a simple pergola with a solar roof. Do bifacial panels do they look good when looking up... Stops plenty of sun and generates energy as well while it's doing it. In landscape most panels are around 1750mm long. So you wouldn't need that many, if you want loads do them in portrait.

That is pretty flat, our house is around 10m at the deepest and 7m for most of it. So that would be only 1.5m buildup at the most. Ours was a 1:1 slope.

Yep that should be the aim, I suppose by coincidence showers and baths tend to be away from the doorway, so win win

Scottish building regs point you to having them at the source of steam either over shower or bath. We have then directly in shower space in two rooms and above the bath in another, but we use the shower in that room most the time. Both layouts work well.

The Swiss are doing it.

I did a similar build in our summer house, the floor performance for a given temperature (same flow temp as the house was pretty rubbish. I would screed or self levelling compound over to get good contact with the pipes then glue a floor down, any air gaps act as insulation.

We constructed our frame in 90mm square hollow tube, it is around 3 to 4mm wall thickness. Then we attached timber to the outside face and the windows sit in the wooden frame and in front of it. As the frame attached below insulation level its a thermal bridge so needs to be insulated internally

Leave it pressurised, thats the whole idea of it. There will be zero issues waiting a week or two if required

If its foam insulation no issues, if as said a vacuum insulated one, bang goes the insulation

not sure you want to do it with tap water - first it could be too cold and then have dew point issues and your bedroom ceiling landing on you during the night was its made lots of water and made the plasterboard weigh a tonne

If you mean under the insulation, then why not - they do ceiling and wall cooling solutions, your just doing ceiling as a retrofit

Was the driver for us getting a heat pump - started with gas. Never looked back, bills reduced massively

I would just buy an express version make life easy. Do you have a filling loop? If so you inject directly in to there. Do biocide and inhibitor from same company and range to ensure no compatibility issues

That's went my plan started to unravel - ended up with nice thick wool carpet in bedrooms - nice insulation for low temp UFH. If there was a next time fan coils for bedrooms and maybe elsewhere

You are correct. But either way wouldn't be doing a build part underground, if I had any other options. Too many variables to start costing you big money.