Mr Punter

Yes and it is a crazy price once you have a special mix. Even more if you want it signed off by Sika or whoever. I would say voids and honeycombing are more of a risk as normal concrete, especially if it has a low water:cement ratio, is fairly waterproof anyway. For quality concrete, specify correctly, don't let the pump guy or concrete lorry driver bully you into adding water and let the batching plant know you will be doing cube tests.

I just looked up what these mean. The top 2 are about 1mm out and class 1 allows +-0.9mm over 8m, so they may be OK. The third one is way out but may be kept for [other reasons]

Leave it until the rest of the work is done, then take a view as it may get damaged or it may be OK as is.

If you are doing polished concrete, don't bother with a mat well. Just use the thin rubber backed mats - they are less than 4mm. You will not need to cut them to fit the well either.

A bit like fixing into Weetabix. I like Fischer fixings. They might work well if you drill a nice clean hole the same diameter as the shaft of the fixing. What do the wood fibre manufs suggest?

There are lots of ways to approach this and as it is a garden room, not a Passivhaus, there should not be any issue with what you have done. You can just lap the VCL over the DPM. Staple in place. I like to leave plasterboard 15mm off the floor, so if you get spills they don't get wicked up. I like the Screwfix pink fire rated gun foam.

I like to tape around the outside as well with the foil tape.

I think some aluminium, zinc or lead flashing round the bottom would work well. You could fix this to the Durisol. It will finish the base off nicely. You could have it run behind the bottom batten.

It does not look as if there will be any issues with your drainage as long as there is a fall from the house to the connection point of at least 1:60. On another subject, the drawings are fairly "pocket door tastic". Although they seem a good idea they have may disadvantages, such as inconvenient to operate, do not open full width, poor soundproofing, poor wall structure and difficult to fit, service and maintain. Maybe have a rethink.

Don't make any concessions to the planners until they mention what their concerns are. No point in obscure glazing if you don't need it. Bear in mind that you may need escape windows in the bedrooms, so an obscured and fixed shut planning condition may not suit.

The perimeter insulation goes on top of the floor insulation. You can make it higher than the proposed concrete level and trim it later.

The salaries seem fairly similar to lawyers and accountants. Do you base fees on roughly 1/3 each for fee earner, office admin and partner profit?

Planning consent should not be an issue because of PD rights (unless they have been removed or are restricted), but to be outside of building regs it needs wall / doors between it and the kitchen, which you don't have and over a metre from the boundary, which it is not. This is an extension and you will need to comply with all the regs. Most onerous may be part L because you have a lot of glazing in relation to floor area. This will mean the glazing would need to be extra energy efficient and you will need some calcs to prove the heat losses are not excessive and possibly make improvements elsewhere to offset. Regarding the floor, this will also need to comply and since it is now part of the kitchen it will be important that there is no settlement between them.

Solid sub base (excavate and MOT if req), sand blinding, DPM, Celotex, polythene, heating pipes clipped down, reinforced screed.

@MikeSharp01 I did not mean to shit you up and I think your solution as originally proposed will be fine. Most condensation issues happen on flat roofs, not walls.