Mr Punter

If they are not carrying current I don't see what this would have to do with BC. If they are compromising a fire rated ceiling or floor you would need to fire stop them.

I don't think you are missing anything. The way buildings behave can be very complicated. Building geometry, local climate, weather, occupancy lifestyle, heating, ventilation and insulation will all impact roof condensation.

+1 I have used these and they work well. You do need a perimeter edge to retain the paving but that is the case whatever you do.

A while ago my next door neighbour locked themselves out of the house. After a while it was decided to break a window at the back to gain entry. I was voted for the job. It took over 5 minutes with a hammer to get through the triple glazing.

Fermacell is Gypsum based, like plasterboard. It is not as brittle as cement board. No special requirements for tiling as far as I know. You have done well if you have managed to line the whole place with Fermacell as it is heavy and difficult to cut and fix.

You should keep your patio level 150mm below DPC to avoid damp issues. Patio level with internal floor level is asking for trouble.

I think coring a hole and installing as per your drawing will be good. I have not seen these before but they look decent.

Any less would be bouncy and not reassuring to walk on. Rafters would need to be 400 as well.

It will be fine if the rainwater connects into a combined drain. I don't know anyone who has ever changed the limestone chippings in the soakaway version.

Cavity barriers and firestopping is often left to "others". They often get left out as nobody really knows what to specify or order or fit. Were they specified and detailed on the architects drawings? Rockwool would be a decent choice for non-vented and Envirograf for intumescent, vented rainscreen applications. I would want detailed spec and drawings, so nothing gets missed on your order or on site.

I really like the XPS backer boards. Lots of thicknesses, lightweight, easy to cut and fix, waterproof. Fix with adhesive foam or screws and washers.

Can you clarify if you intend to extend the existing ground floor, also adding a basement under? Without a floor plan there is a limited amount anyone can suggest. I think that having a measured survey of the existing building and plot would be a good start. Have it include a section through the house / plot, neighbouring properties, front and rear elevations. Budget £1,200. Make sure you own the rights to use the plans.

If the rooflights are toughened and laminated the requirement for balustrade may be reduced anyway. Don't forget access for cleaning with all that glass.

Well that is good to hear. My experience has been that they have been difficult to deal with, I could not get the online part to work. They often want to send out a salesperson but I had issues getting prices. I would like to use them, as they have a branch near me but they are not easy to do business with.

A shame because I have found Howdens arseholes to deal with and have bought about a dozen from DIY kitchens.