Conor

Unrelated, but one of my worst mountain bike crashes was due to a pothole. Towards the end of the Mega Avalanche qualifying, cruising down a short section of connecting road, had one hand off the bar to give it a rest from the brake pump. Hit a hidden pothole, next thing I knew I was about 20m further down staring up at the trees. Knocked myself out, broke my helmet and cam, slightly concussed and had whiplash for three months. Worst part was I was running well in the top 25, maybe 10, and was only 5mins from the end of the run. bike was fine tho 🤣

Is the track higher than the floor level? If so then just pour the SLC upto the current floor level and work gently with a flat trowel. Just make sure the channel is cleared of dust, dampen a bit with a sprayer or wet sponge. No need to prime. Tape over the track to protect it

Self levelling compound.

Anything you put there will be full of roots in a few years anyway. On the plus side, with the hawthorn the ground will be quite dry. Id just backfill with 12mm clean stone.

Need to see the whole floor layout and the ground floor for context. And don't be afraid of moving stairs, not as difficult or as expensive as you might imagine. In your first drawing you're losing a lot of valuable space to corridor.

You're not a proper DIYer until you've put a foot through a ceiling.

Increase the floor build up by 50mm to 200mm - 150mm insulation, 50mm liquid screed. Then either live with 50mm lower ground floor ceilings, or raise everything up by 50mm, which nobody will ever notice.

We had that. Renders put another skim in the affected areas, not noticeable since.

Why do you want to clear it? Let it go wild and maybe stick a few down native trees in and let nature enjoy the space.

I looked at this. Yes, you save the equivalent of a couple courses of bricks. So what? The cost came out the same, was no faster, was a niche skill that needed additional design and prep. You'll want a suspended ceiling for minimising sound transfer anyway.

Just use a layer of 50mm PIR on the bottom, with 100mm on the top. Your local merchant will have 1000s of these in stock, at good prices, as that's what the building trade uses. They won't have any /much 75mm PIR or EPS. As a consequence , 50+100mm will work out better value than 2x 75mm. Thin EPS is a nightmare to work with as it just snaps. Foil faced PIR holds together better, can be cut more precisely, despite it being "harder" to cut than EPS. You also can't have steps in your inusaltion, so just allow the screed to take care of those level variations. (Our floor is 50mm eps, 50mm PIR, 100mm PIR, 50mm screed. 50mm eps at bottom as we changed from 100mm to 50mm screed last minute)

That's a lot of glass for one room, esp with the roof light. you meeting regs with that?

NIE won't allow any of the suggestions above. At a push, they will allow a cavity meter box installed into a wall attached to the house, but within 2m of your consumer unit. We were in similar situation with our ICF build. Architect suggested either fitting in the return wall, or by building a false wall type protrusion against the house and flashing it into the main wall. First option was a no as we didn't have the return wall built, and the second option would potentially cause issues. In the end, as I added 100mm additional insulation, we had the depth to cut out insulation and fit a standard cavity box against the concrete core. A minor cold bridge in the grand scheme of things. For you, I'd suggest the return cavity wall, if it fits in with your landscaping plans.

Looks like water is running down the pipe from above. Likely an ill-fitting tap letting water down from splashes in the sink area.

We had to build over a shared pipe at our last place. It was original 1950s clay... we added an inspection chamber upstream of the foundation, and replaced the 8m or so of clay pipe with PVC. Its not difficult or expensive. A diversion may be more problematic if you have to introduce bends and changes in fall.