Mr Punter

I think you need to get in touch with the people who did the tests and ask why they decided on 1.8m.

You can get double sided butyl tape for sticking DPM / DPC / radon membranes. Use that. Don't overthink. This is just to comply with regs. Anything built before the 1990s has nothing at all, so that is at least 85% of all housing. Use youtube to have a practice doing the corners. It may help if you ever do flat roof membrane in future.

18mm OSB painted white? Easy to fix to.

I think you may need additional funding to stop the project running into the sand. If you share some figures it may help. You will need to get valuations that are in excess of the amount you have spent, so including a proportion of the future finished value may help.

+1 The extension only gains about 4m2 and you have to mess with joining ceilings and floors, plus insulation continuity. Nightmare of a job.

External shutters only work with inward opening or sliding windows.

You also need to consider how to hold them open and secure them closed, gain ventilation as well as if they would be an issue should a fire break out.

There is at least 1 structural wall to the WC and it will be supporting floors and / or walls above. Not a big deal but needs a structural engineer to look at. The second kitchen looks like an add-on. If it is not a properly built extension it may be better to remove and start again. It is a bit of a hotchpotch at the back and £10-15k does not buy you much nowadays unless you are keen on DIY and / or have family in the building trade. There are some architect / designers on here who may chip in.

With rainscreen cladding and a strongly ventilated cavity I have heard that insects don't like to live in there.

I have found that if there is an existing driveway and dwelling, unless your proposal makes the arrangement worse, highways, planning and the fire service don't get too busy with it.

Brick acid? Do a test patch first.

Thanks for the update. I hope you are soon recovered.

With water consumption, we normally fill out a spreadsheet with the proposed fittings and their maximum consumption. Does not require a legal agreement.

Well that is great news for your project. Although you may find siblings tricky, at least they know your foibles and are not such a PITA as contractors can be.

So when you say all the openings are the same, do they all have a crack running down from the bottom left / right edge to the floor? Nothing at the top? Ground and first floor?