ProDave

If you want minimum price per square metre (and wile we are at it, minimum heat loss per square metre) that design is NOT it. On such a large site, I cannot imagine it is the site making you build to that inefficient shape. If you wanted cheaper build cost, I am surprised your ex architect did not mention this to you? P.S we only tried to have dealings with an architect once and his estimated build cost was twice what it finally cost us, for almost the same house.

YES. I have a pretty standard 4kW panels and 3.68kW inverter, but when I submitted my paperwork (pre G98, I forget which) I got an almost instant reply to disconnect it immediately and apply for approval (what would have now been G99) And telling me there would be a fee for even applying. In my case they had seen the inverter model number which contained "4000" and assumed it was a 4kW inverter not 3.68kW and were demanding prior approval, even though that would only have been slightly over 3.68kW. Sending them the paperwork for the inverter confirming it's output was limited to 3.68kW sorted the issue, but it showed there is no leeway. To date in over 3 years I have only exported just over 300kWh. the fact you don't intend to export is irrelevant, the fact is you could.

Yes interesting project. It would make a change for such a building to be something other than a very cold impossible to heat stone building. Net zero is easy. Any house with enough roof space could fit enough PV to generate in a year more than the building uses. It would NOT mean zero bills or off grid because most generation would be in the summer so you would be exporting and most usage would be in winter and you would be importing.

A self builder near me had this discussion with BC and won. It boiled down to the distance to combustibles thing. A stove where they guarantee no more than 100 degrees at the base still needs 12mm to combustibles Presumably to cater for hot ash falling out when refueling?. It does not need a constructional hearth. The self builder argued it was 10mm tiles then 6mm tile adhesive then a floor screed so the combustible issue did not arise. BC failed to find any reason such as a step so passed it. In out last house we had a constructional hearth and tiled finish and wooden floor in the rest of the room and I deliberately set the tiles level with the wooden floor. Again BC muttered about wanting a step but when asked to show me where is says that, he could not and passed it. The 12mm thing allows you to put a 12mm sheet of glass onto a wooden floor and that will comply. In this house we have a 20mm thick slab of granite on top of wooden flooring.

Is this an electric under floor heating mat? What insulation is under the floor? What is the present heating system?

Every manufacturer of a stove will publish the clearance distances from sides and back to "combustibles" and it will vary from manufacturer to manufacturer. For instance our Medip Stoves Churchill 5 convection model requires only 150mm from side to combustible materials and 100mm from the back to combustible materials. It is 550mm wide so in an opening 900mm wide the sides could be combustible material, i.e. plasterboard. The 5kW thing is irrelevant for you. A stive under 5kW may be able to get away with no combustion air provision and just draw air from the room. You are proposing a room sealed stove with combustion air direct from outside so you can fit any size stove that is appropriate. This is the current version of our stove, changed slightly from the one we bought several years ago https://www.mendipstoves.co.uk/churchill-5-convection-logstore-dual-control-ecodesign-woodburning-stove

What hearth and how laid?

You don't need internet for that. Our heating programmer has a "holiday mode" where you tell it the dates you are away and the heating is off for those days. We set the return day 1 day before we actually get back to give it time to heat up.

Even fully modulated down it would be too much heat for our house even at -10 outside

The point is, if you buy the kit and install yourself, it will probably still be cheaper without the grant, than getting the £5K grant and paying for an MCS install. A monoblock ASHP comes charged with gas and does not need an FGAS installer.

Possibly but I am sure before I have seen if you use a waterless trap you don't have a tundish. Anyone more knowledgeable care to comment?

More likely a pressure relief from an UVC could let off a lot of steam (literally) and it NEEDS an open vented stack for that to escape. A waterless trap won't allow it to vent. A 22mm copper pipe exiting the building seems the lesser of 2 evils compared to a vented 100mm drain stack.

What is the build standard of the rest of the house? i.e. standard, or all air tight membrane and service void? A cold loft is the hardest thing to detail as not only cracks like this want sealing, but EVERY penetration, every light fitting, everywhere cables drop down into a stud wall etc etc.

Why not post some pictures of the pipework and what connects where, it might give someone a clue.

We have dishwasher into the kitchen waste and washing machine into the utility waste. It is true, that when one empties, you hear the water running out of it and down the sink trap. I assume this is what @Nickfromwales refers to as "gurgling" When the appliance is not in use it makes no other noise. Personal preference if that noise bothers you, but if that noise bothers you then chances are just the noise of the appliance also bothers you. We have a dishwasher in the kitchen as that's the obvious place but I would regard it as a VERY backward step to have a washing machine there. That is what a utility room is for, tucked away so the noises of the stuff in there don't bother you.

The advantage of an ASHP bigger than needed for space heating, would be quicker DHW re heat times. But it would not be able to modulate down low enough for heating needs so would indeed be cycling more.

Different people do things in a different way, but typically on a cylinder with a WBS I would have two tank stats, one to turn the boiler off when it reaches set temperature, and a second one, set slightly higher, to turn the CH circulating pump on (regardless of programmer settings) when the tank starts getting too hot from stove input, to dump heat to the radiators.

Check the BOILER thermostat. If that is set below 75 degrees then you will NEVER get there from the boiler. Often boiler thermostats have no scale, just min to max and arbitrary units. Post a picture of the boiler and it's thermostat. The thermostat on the tank is probably * what turns off the boiler when the tank reaches the correct temperature when heated by yhe boiler. The thermostat on the pipe is probably on the flow from the stove to turn on the heating pump when the stove is producing heat. * Probably as dual feed tanks particularly involving a stove are more complicated as the stove is an unctontrolled heat source.

But following Brexit, can you just import a Spanish VAT paid item to the UK? Surely now the Spanish supplier would have to sell it ex VAT as it is leaving the EU and then you pay UK VAT on import (which you later claim back). That is probably why they are looking for a VAT registered agent to handle it?

Here would be an interesting experiment for someone with lots of time. Take a bit of ordinary t&e cable, load it to it's maximum rating and measure it's temperature (some form of probe attached to it) Then bury in in the middle of a load of EPC beads no more than 100mm wide (or whatever your cavity width is) and repeat the load test and see how hot it gets. Wiring regs talk about cables "in insulation" But there is a big difference between a cable enclosed in the middle of 300mm of rockwool and one enclised in the middle of 100mm or less of EPS beads. Yes you could derate a lot of the circuit breakers, but not on things like cooker or shower circuits. I think the issue is not overheating and increasing volt drop, but overheating and melting the cable. A rise of 50 degrees C would be very bad for PVC.

Sorry I am thick again I see the socket in your picture and the plug that has to go into it and see no reason it just won't plug in. Just what is in the way?

Remember the level entry requirements stipulate a flat area in front of the door, so a wooden ramp on top of that, would have to have that flat are first before it starts to slope. That is NOT a "level entry" door. If it was it would not have an external cill like that, just a threshold strip that floor inside and ramp outside tuck under.

Then I would NOT fill the cavity with any sort of insulation.

In the kitchen picture (with the loo roll mountain) what is the purpose of that illuminated recess under the worktop on the island?

I would have closed them with noggins where they crossed the outside wall, then insulate up to the noggins on the inside.