Mr Punter

I don't think it looks too bad. I think you could leave it as is and it will stay standing for another 20 years.

Before you go ahead, check with your warranty provider and building control that they will accept ICF in a basement. They can get quite jumpy about it.

Quite a few bricklayers would just stuff the insulation in any way round. Any cut required would be made with a trowel. I have specified the rigid insulation because on paper you get a decent u value for the wall thickness, but I would go with wider cavity with poly beads or dritherm 32 slabs in future because the rigid stuff is such a pain to get right.

They may just mean 8% annual interest accruing daily. Otherwise they are idiots.

Yes you can have it in the cavity but it will be a cold bridge. Best to have a bit of insulation around it. You will paint the bottom metre or so with bitumen paint. No issue with DPC. If you have a cavity tray running around the wall you will need to stop it either side of the steel.

I think there may be issues with his account that prevents this?

You could do 100mm blockwork in the middle, resilient bar and 2 x 15mm DB plasterboard either side. About 200mm total. You may not need thermal insulation if both sides are normally heated.

You must make sure the new covering achieves B Roof(t4) fire rating.

The doors will sit on and be supported by the outer leaf, just overhanging by 20mm.

Can you run the 150 pir to the outer leaf?

Just to add I have no legal training or background but I have had experience of not serving a Pay Less Notice under an NEC3 contract which made our position very weak.

Just get as thick a wall of insulation as you can around all the hot pipes. Cable ties are useful.

Yes. Don't. Do it during normal working hours and use dust suppression.

I think yes to the Pay Less Notice as it is a formal method of recording what you have done and accords with RICS, JCT, NEC type contracts. If this goes to Court or adjudication you will be seen to have followed best practice.

I had a plot that had planning including green roof. I went back to planning to get it changed. I did not want the associated risks. Fire, weight, appearance, maintenance, insurance and mortgage / resale were factors. We had standing seam aluminium in the end.

Just pay what you think is due and explain why. Depending on the contract, you may need to serve a Pay Less Notice, which is a simple document that you can download free.

Maybe. If you need to compartmentalise the walls, say at each floor or junction with a garage, you may need fire barriers. With a ventilated rain screen system this is sometimes at odds with the need for ventilation behind the cladding. Vertical barriers can be done cheaply with rockwool socks but sometimes you may need expensive intumescent horizontal barriers. It is difficult to find definitive information on what is needed where and how to install it. It just gets left to "others".

Did you discuss / agree this as a variation, or were you unaware? Bear in mind if the extension is 3 sided and the 4.6 is on the end, they have done 10.6 l/m instead of 11.0 l/m of wall, so about 3.6% less. Most of the costs would be the same, unless they reduced sockets, radiators, kitchen units etc. Prelims would be the same. You could use it as a lever to get a bit of paving done. If you think they did a decent job, pay up.

Was there no way the slab could have been left?

I agree the Viessmann looks good but I wonder what the average actual savings are for the high modulation range given they are about £500 more expensive?

It won't look silly to have a bigger step. The other option is to move the IC further out, but it may be quite a lot of work.

You only need to size the boiler for DHW. The heating demand will be covered several times over.

I do not think a build over agreement will apply.

If you are not building any new walls or foundations this should not apply. Changing use will not affect the integrity of the drains!

The warm roof / cold roof is confusing on a pitched roof, where it often means to refer to the loft, not the rafters. Almost no pitched roofs are truly warm, with all insulation above the rafters. The architect has specified insulation between rafters (I assume fully filled), breather membrane above, counter battens and battens, then new tiles. You won't need to touch the ceiling.