Thorfun

you can get Phenolic insulation with a thermal conductivity of 0.019 W/mK but it is a lot more expensive. then you can get vacuum sealed insulation but it is stupidly expensive and can't be cut and any penetration will negate it's thermal properties so i wouldn't consider it personally. you'll need to speak to the manufacturers of the phenolic stuff and see what would be suitable and then perform the calculations to see what the U-value will reduce to if using it and if the cost is worthwhile for you.

A timber frame

yep. it's a pretty simple solution albeit with lots of layers. I could've done it a lot cheaper myself but I wanted a guarantee for the flat roof covering (was also required for our warranty) so I got a professional to do it all. from insulation to waterproofing to green roof to edgings etc. including the hire for a telehandler was about £400/m2 to give you an idea. so, as I say, £100/m2 seems pretty cheap.

£100/m2 seems quite cheap. Does that include insulation, waterproofing and the green covering or just for the greenery?

isn't that what ditra matting is for? (other decoupling mats are available) our tiles (the entire ground floor pretty much) was laid on a cement based liquid screed with a decoupling mat. tiler had no issues doing this.

starting again with the "i'm no expert" caveat...... it is my understanding that a sand and cement screed will take a long time to dry out regardless of the cement you use (1mm a day iirc). i'm convinced that your floorer is talking about liquid screeds as anhydrous liquid screeds take a lot longer to dry out than cement based. if you use a sand and cement based screed you'll still be waiting a long time for the floor to dry out before you can fit the finished floor. i would double check with your floor fitter as to what he's referring to when he's talking about non-anhydrous

afaik from my research you can get anhydrous and cement based liquid screeds. i didn't want anhydrous due to having a laitence removal requirement and being gypsum based. the cement based liquid screed was brilliant and ticked all the boxes for me. @nod is a screed expert (i most definitely am not!). hopefully he'll be along soon to comment. 😉

cemfloor here in our basement. can't remember the cost off the top of my head but it was miles away from £9k!!

this was my understanding too. are you sure he's not thinking of liquid screeds?

i'd say that was a pretty unanimous response! 🤣

ps. have a read of this thread as the OP was also told couldn't have an insulated raft due to ground conditions but their SE made it possible.

can't you just use normal block and beam and put loads of insulation on top? 🤷‍♂️

ps. here's our blog post on the basement ufh if it helps

we didn't put UFH pipes in our slab in the basement and i ended up putting between 25mm PIR on top of the slab and then fitting pipes to that and putting a 50mm liquid screed over the top. seems to work in our house. on the ground floor we have block and beam over the basement and insulated slab (EPS below the slab) on the 'arms' and put 50mm - 100mm EPS on top and then a liquid screed on top of that. house is lovely and warm so it can be done. as you're going PH i don't think you've anything to worry about in having the UFH pipes on top of the slab with a screed on top (if you ever need to turn it on that is!). just make sure you allow for it in your design so that your FFL is correct!

And I’ll have the external blinds automated by then too so as to reduce the impact of the solar gain.