Mr Punter

I have done quite a few small infill plots. You may be able to go down to 3 boards wide, so 750mm.

This never ever happens. In the skip and replace with new. Rooflights are often toughened and / or laminated, so even if you wanted to endlessly faff around it would not be possible.

No. Render and paint please.

I think the SIP sheathing wants a 25mm minimum clear cavity, so no.

I have just had 12 windows fitted for a similar price but new build so nothing to remove. Also we are left with any packaging / general crap.

If the blockwork is being rendered you could sort it with a render bead. We have rendered the underside of lintels by prepping with rend aid.

Maybe this sort of thing then https://www.tcfixings.co.uk/product/simpson-1200-x-30-x-2mm-light-restraint-strap-bent-100/16842

I am struggling to see what the issue is.

If the window is not square and level it will need refitting. What is the exterior finish and is it complete?

It is a real pain to apply and very messy. You need loads of coats to meet the required thickness. It goes manky if exposed to the elements / condensation. You will struggle to get a decent finish.

I think you have labelled the vertical timber battens incorrectly. The arrow looks to be pointing to the 30mm pir. Regarding holding down, you could use https://www.metrofixings.co.uk/product/simpson-strong-tie-htt5-403mm-hold-down-strap/23994 fixed to studs and bolted to the slab internally.

Yes I am pretty sure. That is why housebuilders don't use it. I doubt it would get a warranty or mortgage as a dwelling.

Would a bottom outlet chamber work? https://www.drainageonline.co.uk/underground-drainage/drainage-pipe-and-fittings/marley-underground-drainage/250mm-diameter-double-bottom-outlet-chamber-body

You may want to consider aluminium copings for this.

No. If it is going to support roof and floors it needs to be structural steel and will cost much more than brick / block, timber frame, sips or icf.

You may find that improving airtightness will be more beneficial. What is the floor and roof insulation like? I am just thinking that ripping out the plasterboard will be a bit disruptive. If you do end up doing IWI you could use a PIR board which won't eat into the room as much.

So when you need a plasterboard, you take the clip off with a drill, move the board from the stack, then reattach the clip. Maybe cone off the area and set up some flashing lights and a siren. And have a couple of security guards on hand.

Once the house is completed it is very difficult to tell the difference.

Just lean the boards at a bit of an angle.

A couple more quotes and you be able to decide.

Can you rotate to get the fitting nearer to the top of the soil pipe? Maybe a bend with a spigot and socket would help? Hopefully someone who is decent at plumbing can make a more useful comment.

Was this galv and powder coat? It looks like you went through the wall for the fixing rather than resin. Did this need SE signoff?

It looks OK. 25mm gap will work.

I had a look and they seemed to deal in board products. Were the wall ties BBA or equivalent?

There is a cold bridge but not the end of the world. It may be worth a bit of insulation over the UFH pipes where they run down the corridor if it is not too late.