Mr Punter

+1 OSB for racking, breather paper, battens, cement based siding / cladding boards.

Will they accept a professional consultants certificate? Some of these can be issued retrospectively.

NHBC like a drained and ventilated cavity in the construction. I can see why.

Screw a large piece of ply or osb behind the rad, into where the studs are. You can then screw your rad brackets on where they need to be.

I have looked at their other posts and they are having issues getting a fixing for the rads! https://forum.buildhub.org.uk/profile/3234-phloaw/content/

For sprinklers in the building our contractors used Blazemaster pipework. 85% of the cost is for design and first fix. You really want it tested before you plasterboard. It is good to get a separate water connection for the supply.

That was a warm pitched roof, in the meaning of insulation between and under rafters.

The water will get in through those hairline cracks. Perhaps a low modulus sealant like https://www.screwfix.com/p/no-nonsense-builders-silicone-grey-310ml/912gr

£9,384.00! At least there is free delivery. Is there much benefit in having the hybrid compared to a modern ASHP and oil boiler separately?

Not sure how the feature stone is supported? Do you need a single leaf l-shaped lintel in addition? Could use some galv or stainless angle for the outer leaf?

I think @Gone West has a decent idea. Demolish the lean-to and build another extension the same depth as the shared one with a valley to link the roof.

If the policy is to get people to switch from gas to electric, they should be increasing the cost of gas and perhaps lowering the price for electric.

Just a small fascia board. Choose any gutter you like the look of. You could have stainless or copper. We have black powder coated ali with small fascia.

I think you could put some Lindab aluzinc gutters on and lose the soffit overhang and it will be fine.

You could get the BCO signoff before the wardrobes go in.

Could the windows be supported off a steel beam with pad foundations?

Can't you put a small hatch to the right, under where you have had your pocket squirrel out (again)?

https://www.fhbrundle.co.uk/ and https://swr-balustrade.co.uk/ may help

Would a continuous dry verge help?

It will take too long to respond as lots of heat will go into the slab. A small amount of insulation on the slab would work.

With our schemes we normally have a full set of planning drawings at 1:100 that have been produced in CAD format. You would need to decide on the build method, then get both architect and SE to coordinate with the design. Architect should have a decent idea of what to expect as many components for a house are standard and known to them.

If the additive is not approved for this, why is he using it? I thought that normally you would get readymix for ICF, not site batched.

If I were your neighbour I would not be happy with a 2 storey flank wall 3.3 metres away from my South / East facing corner feature window. It sounds like they will allow you to have a 2 storey extension, but no closer to the neighbour. You could probably also have some single storey element. Are you unable to get satisfactory accommodation within this restraint?

165m seems a long way. I assume that none of you neighbours are connected to mains sewer?

Well those costs seem to square with @IanR and are quite reasonable if they do a good job. Sadly this is not always the case and I have had experience of the job being rushed, wrong materials (cheap substitute for Compriband etc.) glass and powder coating scratched as the units are manoeuvred etc. As ever, if you are lucky enough to get a firm that are professional, skilled and at reasonable cost then you would do well to use them.