Definitely sand first, the dust gets absolutely everywhere. Ours was decorated first although not the original intention and although the guys did a great job of clearing up every now and again I find little bits even high up on walls that I've missed. I also had to have little parts of the skirts retouched up with paint

When we applied for our first floor extension over our garage / family room, our architect was very clear that the extension would need to be "subservient" to the main house. He achieved this by stepping the front back slightly, only by a couple of bricks, and consequently the roofline is slightly lower than the original part of the house. However, the side wall was not stepped in. So, it might be worth seeing if you could maybe push back the front of the first floor, whilst keeping the side wall as wide as the ground floor, and reapply.

Hi, It's a decent plan overall, maybe shuffle the kitchen around a bit, it seems a bit small to me and the layout is not perfect. Try rotating the island, see how that works for you. Also maybe use as much as you can for worktops and storage, especially that you want to use part of it for access Annex - Bed right next to utility is an arguable solution. Also there's no window nor natural light or ventilation to the bedroom. Good effort but try to play a bit with these, see how it goes. My humble opinion as a pro.

Hi we have moved kitchen to back of house and laid UFH albeit on concrete base. Floor needs to go down first, blank panels under where the floor units and any island units will be, then a thin screed on top before laying something like Amtico or other lvt. We laid parquet flooring and had to follow the same procedure. To save money and time laying the parquet under the units as we did it all ourselves we put large pieces of osb on top of the screeded blank panels (where the floor units were going) so that there was a flat surface for the units to be fitted. You wouldn't have much depth on LVT so could use marine ply perhaps which is a lot cheaper than Amtico under your units.