ProDave

Two obvious ways. Design a house that works around a central stairwell hall and landing, but only build to one side of the stairwell, add the other side later. Make all the future openings as doors or windows for an easy knock through when the time comes. Or design a 1 bedroom bungalow but with the roof structure already built using attic trusses so converting the upstairs to more accommodation is easy.

That's fine, you can notch them, just check you put the notches in the prescribed zones. I must have got confused with another thread and thought you had posi josts, sorry for the confusion.

See my post above. At the other end of the run, the pipe must cross over EVERY joist.

Your proposed idea gives you one MAJOR problem. How will the UFH pipes cross over a joist, which they will to at one end, crossing over every joist. You cannot notch a posi joist. I did it this way on a very small section of floor under the bathroom, and I threaded the pipe between the top and bottom chord at one end, but it was VERY hard work with only a short run of pipe to keep threading through. If you are going to be doing anything like a normal sized room, you could be having many tens of metres of UFH pipe and threading it through again and again is going to be somewhere between extremely hard, and impossible. Battens along the top with a small gap at one end for the pipes to pass solve all this. Set the celotex level with the top of the joists.

Okay as someone who did this (a little different) I wanted to be clear that you are putting battens along the top of the joists to support the floor, with the biscuot mix filling in between as a heat spreader, but not supporting the floor. This solves the problem of how the UFH pipes cross the joists, you leave a small gap in the battons. and it means you don't need much support for the celotex. I take it the joists have been specified taking into account the extra dead load they have to support.

I think a lot more detail is needed. Will this be the Celotex with the grooves in for the pipes to fit flush? or will it be as someone else suggested be covered with a biscuit mix?

It was my relatives in Queensland that told me about the self build permit. They even had to get one to renovate their old derelict Queenslander. It may be different in other states.

Start by moving to Australia.

Re the siting of isolation valves. I wanted each appliance to have it's own isolator in an easy accessible place. But I wanted to achieve this without making the pipe runs, especially the hot pipe run any longer than they have to be. What I settled on was a central distribution point, which has ended up in the space above the ceiling in the utility room. This puts all the isolators together, and it will be finished with a mini loft hatch to lift out to get to them. As this is just the utility room I don't mind having this little loft hatch there.

Not the Australia I know, where you have to do a course at college to get a self build permit.

I fixed ours direct to the timber frame. The logic behind that was only half the room is done with multipanel. The dry end of the room is plasterboard, that too is fixed direct to the timber frame. So there is a level transition between the multipanel and the plasterboard.

Well it quotes a power of 150 watts per square metre. You say the room is 3.5 square metres so that would be a power of 525 watts. At a typical electricity price of 14.8p per KWh that will cost 7.77 pence per hour that it is on. The rest is guesswork as nobody can tell you what duty cycle it might run at to maintain a particular temperature.

Forget the uphill / downhill bit. This is mains water under pressure, the end point is the same regardless of how it gets there.

BC will also want to witness a pressure test. Buy a pressure test kit e.g this https://www.bes.co.uk/drain-pressure-test-kit-4074 And a few of these https://www.bes.co.uk/drain-pressure-test-plug-4-110mm-1-2-test-point-1960

The one you linked to has an overflow.

Yes, middle knob is temperature. Top knob is flow to the overhead rainfall shower head, bottom knob is flow to the shower head on the riser rail. You can have both together if you want.

On our last build I hired a big 4WD swivling dumper. Fully loaded, it would not climb up the slope out of the trench, the digger driver had to give me a push. You would want it to be very lightly loaded for that. I would be more concerned if it got stuck not to let it turn sideways on that slope.

Simple.

Mowers were designed for petrol engines, not to be constrained on the end of a leash to get in your way,

...... And there is more to go wrong. If you must go electric I would always go Triton for value for money and reliability. But No 1 choice would not be electric, but decent thermostatic mixer from decent WW tank.

Major flaw there. What stops the water running off the 3 ridge tiles from running UNDER the top layer of felt that seems to stop short and not go under those ridge tiles?

"free Ceiling insulation. BUYER COLLECTS"

Typo corrected. I meant -10, with 30 degree difference between in and out. Though one night this winter it did get to -15

@JSHarris has produced a spreadsheet to model heat loss from a building and I can certainly say it gives accurate answers. you input your house details, wall and window U values and it tells you things like KW of heat loss at different temperatures. It accurately predicted the heat loss from my house when it is +20 inside and -10 outside is a shade over 2KW I am sure someone will be along with a link to download it.

Nor mine. That was a bit of a surprise when it came in last year (or was it the year before)