Mr Punter

I think you are out by a factor of 10, so maybe you meant per square metre?

It looks like there is no skirting. Normally the plasterer leaves a gap at the bottom as it is very difficult to plaster down to the floor, keep it flat and not spread crap from the floor. I think this is why it is flared out a bit. The waterfall edge can be scribed to the wall. Probably easiest to do in the workshop.

Another vote for the brush in stuff. It can get expensive if you are doing wide joints or cobbles, where sand / cement can work out better.

You can get an insurance policy for this but I would not bother.

The only issue I could see is if your door threshold is not thermally broken and in contact with the screed you would have a cold bridge.

They should open loads more than that. Pull harder on the handle.

ICF in a basement can be tricky as you cannot inspect the concrete. A groundworks company should be able to build a basement and they do not need to be registered. You will probably still need to use 2 forms of waterproofing. Basements are fairly high risk as the waterproofing can often fail and the cost to remedy can be high. You need a ground investigation and structural engineer.

So slab, 170 pir, screed. Is the 100mm Foamglas on the slab?

If that is the case, just show shiplap. It is only the external finishes that are planning matters. Different for Building Regs, where they may want something non-combustible.

Yes, I was suggesting a replica GRP one.

Best to get the amendment agreed by the planners. It would be really annoying to finish the extension and later have to erect scaffold to fit a GRP replica stack

Polythene DPM is probably cheaper and it is clean and easy to install. If the slab is not flat you could use some sand to level it before you put down the DPM.

Timber frame is good suggestion. I have seen a lot of bungalow to house conversions done this way. Standard factory made ones tend to use 140mm x 38mm cls studwork for external walls, with 9mm OSB sheathing and 89mm x 38mm for internal.

The window looks fairly new. It is quite surprising how much force is needed to open and close these. Maybe get in touch with Velux suggesting the product is defective? They may let you have the part a bit cheaper.

You don't need to alternate the block on flat from inside to outside. Do it inside all the way. BTW Poratherm has many negatives. Very difficult to get a fixing and not good thermally. There is a reason they don't get used over here much.