Mr Punter

Just extend the slab to the external leaf. If you were to do as you originally suggested the screed over the cavity will only be supported by a thin bit of foam over a void. You do not need a cavity in this section.

I would normally expect to pay about half for first fix. Get a couple of quotes for the remainder of the job and forward them to your person. See if he would rather come and finish the work or have you appoint one of the others and pursue him for the money. Bear in mind the original spark could justify some reasonable extra cost for the delays and materials and labour increases.

Just take the inner leaf down level with the slab and fill the cavity with concrete level with the top of the slab.

Get rid of the inner leaf where the door opening is or take it down level with the slab, then run the DPM and foam all the way to the outer leaf.

I think you need 18mm all over. I bought a couple of sheets from Wickes yesterday at £47 each, so I feel your pain.

If the current house will provide you with enough funds, get a mortgage on it now. Mention that part of it is a building plot and you would like their confirmation that they will grant consent to transfer this at a later date. I have just mortgaged at 0.94% fixed for 5 years. Fees were £1000 but valuation and legals were included. The way to have 2 titles is by transfer of part to another party. You cannot transfer to yourself. Is there ANY way you could have separate drives?

I know of a house where they used sheeps wool insulation. Although it was boron treated they got infested with wool moths which were very difficult to eradicate.

It will be a real pain to strip the cladding and tile it. Have you worked out the cost? It may be worth doing this and presenting the council with the costings and time involved to see if they could be swayed. Paying thousands and throwing away perfectly good materials does not seem environmentally or financially responsible for the questionable benefit that may arise.

I think you will get caught for CIL as if they were dwellings.

As you have beam and block you will need the ventilation for Building Regs and probably warranty. Too late for a ground bearing slab. We did a place that had external periscope vents to the basement. They didn't look great but served a purpose.

Would it be worth getting an airtightness test done on the house? It can help identify where the most leaky areas are. A thermal imaging camera may also be useful.

If they were manufacture supply and fit at least you only have one firm to chase. The class 7A was achieved under lab type conditions. Any bits of missing or badly fitted gasket, incorrect adjustment, poor sealing, bad detailing etc can end up letting in water.

I don't think that the air to air heat pumps like @Hanksy has will work in SAP. Resistance heating is fine but punitive.

I have a SAP here for end terrace 115m2 that says the total space heating requirement is 30.89 kWh/m²/year. The December and January heating requirements are about 730 kWh/month. Another is a large mid terrace and 197m2 the total space heating requirement is 27.66 kWh/m²/year. The December and January heating requirements are about 1150 kWh/month. These were built 3 years ago.

I think it would be best to connect your foul drains to the foul sewer and the surface water to the culvert.

So a back inlet gulley on the surface water will trap all the smells on that run. You can tee into this with all you surface.

You should not be getting smells from a surface water drain but if you want to you can use a back inlet gulley between the IC and the rest of the surface water system.

The regs have some flexibility. The worst you can have for a floor is 0.25. The suggested is 0.13. In reality if you are at 0.19 you will need to do a hell of a lot elsewhere. We were putting in 75mm Celotex 15 years ago, but the regs have tightened up a lot since then. Ground floors are an easy win as the insulation is simple to fit and you don't lose any floor area or have a larger footprint. I think you should look at 150mm PIR or 250mm EPS.

If the pir is your floor insulation, 75mm is not much. If you have UFH you will lose lots of heat.

Things may have changed, but it was the case that you could not have A2A heat pumps as the main source of heating so if you specified them for your new build you would not pass SAP.

1 will work fine. Maybe best on the right side so it lines up a bit more with the stud below.

With UFH on the ground floor you needs lots of insulation to prevent the heat escaping. 100mm is not enough, even if it is PIR. Insulate properly, make it airtight and you will not need UFH upstairs.

Can you have a mini pump station near the WC and pipe to the drain in 63mm MDPE? They are almost silent in operation and will probably only operate a couple of times a week. You would need to form a sump for it but ANYTHING is better than a Saniflo.

There are special pavers that are shaped to leave a 3mm gap between. The system requires special fines free sub base and pointing with a kind of grit. All very expensive. If your BC accepts normal kiln dried jointing sand then that will be good, although not permeable / SUDS compliant. https://www.marshalls.co.uk/commercial/permeable-paving-and-suds

If you get the insulation and airtightness right this should be fine. Electric towel rads in bathrooms should be enough. Cables for electric panel heaters is a good plan for when you sell.