Mr Punter

Glued will feel more solid but either will do. Either way, leave a decent expansion gap around the perimeter.

I don't know what the need for the floor to breathe is all about. Suspended timber floors often had outside airflow under to help protect the timbers from rot. I don't think the batten gap on the room side will do any good. 6mm ply is not stiff enough for a floor. You could lay some thin 10mm XPS and overlay with chipboard.

I don't understand the 45 deg angled screws. With the washer head they won't even sit nicely.

Obscure glass will be fine here. It is for light and maybe ventilation. Also you don't need to concern yourself with cleaning it.

It looks like it is flaking. Leaving it may cause further issues. Can you get it done now?

It may be the contamination risk from a leak.

I understand aircrete is about three times poorer, but it is 3 times the height, so by the time you are at the top of your insulation / screed layers they are roughly equivalent.

I have used a course of Marmox and medium density blocks. You could probably get away with swapping the Marmox for a course of 7N aircrete if you want to save some money.

Have you seen any responses from consultees? The planning officer will often wait for all these before they write their report. It may be worth emailing her to establish if she is likely to support your proposal. Forget the 'escalated' bit as it is inflammatory and will just piss her off. Same for the Ombudsman complaint. You need to play nicely.

The top half of the flashing should be under the tiles, not over them.

What drawings were you working to? Did Velux supply a detail for the zinc roof covering?

If that is an uninsulated concrete floor you may want to consider a new insulated floor. Otherwise, some levelling screed.

A drawing may be useful. Is it stone with insulation behind then a frame / blockwork? Is it new build or conversion?

You can't always tell how something is supported until you dismantle. The spindles at the end are different, suggesting the return balustrade is not original. Nothing insurmountable here. Is the end corner of the landing wobbly?

You will need to treat the room as a basement.

Would it work if you cut the zinc down a bit and dressed another material to act as a flashing, like a single ply membrane, over the top of the upstand and down over the zinc?

Well that is a very ambitious project! Good luck.

I understand you would cut a slot in the mortar bed, clean it out, gun in fresh mortar, dip the tangs of the hanger in the mortar and insert. Masonry hangers don't get nailed down through the top.

By no means a disastrous sum. Looks the biz.

I am not keen on staircase open to kitchen on escape route but decent mains interlinked alarms (I like the Aico ones) will mean that you are alerted very early.

Yep

Could you use some masonry hangers onto the top of the lintel and lose the ledger plate?

They do look very good though. Dare I ask the price?

Underneath the doors the haunched concrete makes no odds. No water is getting down there at this stage. I sometimes just omit the inner leaf under doors. Where there are not doors, on site the haunched concrete fill gets filled with mortar from the brickies and sometimes bridges the cavity. Looks good on the drawings though!

Get a dehumidifier, plumb in the waste pipe and leave it running 24/7. If the room is dry you will not have issues.