Hi again , so from the off I’m not a professional builder but ive done several over the years , so my way is as follows - pro builders may not agree but …start with the floor , if you want the two floors to be level allow 50 -75 mm of screed + your finished floor height tiles / laminate / floor etc then add 100mm of insulation and the remaining difference is made up with concrete , dpm is imo reqd but you need to be careful you don’t bridge the external damp course , as for walls any power requirements will need to be considered and of course plumbing cold supply to washing machine / hot and cold to a wash basin or sink . Insulation is but don’t quote me is 75 mm with a 25mm air gap then your plasterboard and skimming on top of this . 3x2 or 4x 2 stud frame with the insulation in between . Bear in mind the insulation/ plasterboard will reduce the available area so any kitchen units / wall cupboards etc are best measured when the walls / ceiling are complete . As I said at the start I’m not a professional so I stand corrected but in essence this is how I would go about it . You may need to seek advise on weather you need planning permission . the door separating the house /utility will probably need to be fd30 frame / door + door closer fitted minimum .. but again others may say different . It’s a decent sized job and the materials cost will be quite high so if you have an account at Selco or any other builders merchant you may be able to get a decent discount .a cost saver could be to use timber / insulation and omit the concrete but it’s probably minimum ., hope this helps and have fun