Mr Punter

I hate plumbing. It is the cause of the vast majority of defects. I think we should bring back the outside WC. The hidden cistern is probably going to haunt me.

Well I have no idea how you would change these in a full tiled bathroom! I have 6 at home.

If they are too tight they will snag and you will lose edges, esp if you need to take a board down etc.

I have rewired places years ago where the vertical drops were in conduit. It made it much simpler than chasing in. For horizontal it was under floor.

I would use medium density blockwork. It is about 1400-1600kg/m3. Your marble will be about 2700kg/m3. With 2 layers of 20mm tiles and a bit of adhesive on 100mm blocks, you will be around 260kg/m2. If you did porcelain it could be 10mm. I would not want to tile onto aircrete blocks.

It may be worth connecting a rocker pipe immediately the drainage pipes exit the raft.

This is the way of things. Lots of back covering and padding. It is not in anyone's interest (apart from, maybe, yours) to get you to underinsure. Shop around as the likelihood of a successful claim for the full amount is probably vanishingly small.

It looks very difficult to reconfigure. Round here they often add an extra floor, remove most internal wall etc when doing a bungalow. Have you looked around to see if there are other properties that would suit you?

The insurance rebuild costs are often higher than the actual build costs. It may take account of demolishing existing and maybe removing foundations and services before starting again, with a generous allowance for fees, prelims, contingency, overheads and profit.

This arrangement happens all the time in England. Conditional Contract or Option Agreement are the standard means. With a conditional contract, you exchange contracts at an agreed price subject to, in your case, obtaining planning consent for erection of a new dwelling etc. There will be other constraints to cover time allowed, appeals, fencing and boundaries, access, working times etc. Once the conditions have been met you are obliged to complete the purchase.

Unless you have someone in mind to take on the second plot, just offer for the one you like best. Major hassle and risk once you step outside the self builder role.

It only seems to add water to bagged mix. If it could properly mix the dry ingredients (gravel, sand, cement, admix etc.) it would be worthwhile, so you could make concrete, screed, render, mortar etc.

True but the AC could be over 10x the MVHR

Go back to the supplier/installer and ask for this. It could be a tag or EN number.

Do the MVHR and just put AC in the rooms that need it. If you have lots of PV on the roof the AC will cost nothing to run during the day.