Mr Punter

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.

They do a smaller merchant size at 2.0m x 3.6m which may work better for cutting up.

Is it too late to move the timber frame out by 40mm? That way you could do the internal Kingspan with no loss of space.

If you get to the end and have, say, a third left over, can you just go for a really big lap, rather than cutting? Often the offcuts seem to end up in the skip or laying around to trip people up.

We had a dedicated 50mm connection for the sprinklers. We did the potable water in 32mm. Do not try to put in a single supply for both or you will get hit on the standing charge for the larger meter. Water cos don't charge for fire supplies. Specialist job 100%.

Drill for dowels first with a spade bit.

We had sprinklers fitted throughout on a recent development and the cost was about £2,550 per house. Average house was 4 storey 170m2.

I will speculate that it makes none whatsoever and nor will it make any difference to thermal bridging if the I joists. It will also be much easier to support plasterboard on this than it will the external tile cladding, which will be far more demanding.

The mist ones cost a fair bit more and do not typically cover the whole building. They are often specified for shared kitchens in student accommodation.

Although there is supposedly good evidence to say that timber frame is OK in a fire, there has been another today. These are far worse in multi storey blocks than detached houses. This latest one was also timber clad, which seems like madness. I assume no sprinklers either. https://www.24housing.co.uk/news/fire-rips-through-timber-framed-block-of-flats/

I have used some of the rubber stuff, but only on a garage. I did it on my own but it would have been much easier with two. There are some here who have installed fibreglass (in bone dry conditions) with great success.

Those pillars are fairly small. It won't take much material to fill them. Half a bulk bag and 5 cement would do both.

That is not the case, it was a bit of an old wives tale and never proven. Between 1:20 and 1:60 seems to work well. I assume there is a downhill slope, so I would go with this and have the drain fairly shallow all the way, providing enough cover for whatever is above. I think you can go 45m between inspection chambers and if the drain run is shallow you can use the small ones.

My guess would be this is a floating floor without a sufficient perimeter expansion gap. Ask the previous owners. You may need to remove the skirting and use a plunge circular saw to cut the boards near to the walls. I have a Makita SP6000J which gets fairly close to the edge, but there may be better. You will need to finish the ends of the cuts with a multi tool.