I Have done many garage conversions ( both years ago in Barry and then down here ) I guess with the Bed, Kitchen, Toilet its going to be a " granny flat " or a " independent son / daughter place " , for this you will 100% need planning permission, the toilet and bed will dictate that

Now, I always ask potential clients at the 1st meeting what they want out of the conversion, plenty just want a Home office, some want a man cave, now this might include a toilet / small kitchen, If they are ok with themselves " bending the planning rules " it makes it alot easier to do, they create a " store room " ( call it what you will ) and convert that, yes that " store room shoudlnt have a kitchen or toilet , but who the hell will know ? ? ? going down the legit planning permission will add a few K onto the price right at the start

By avoiding planning and building regs ( now i am not talking of unsafe work, Electrical work will be done by a sparky and signed of ( if major ) though most garages will have lighting and a double socket on its own MCB, so not massive to extend the electrical ring ) will make the job alot easier and flexible, not to mention cheaper

as for the design,

I will lay down some DPM, on top of that lay either use 100mm or 75mm X 50mm " ( more often than not C16 CLS ) floor joists, with 50mm PIR board ( celotex / kingspan are 2 fairly common brands ) between the joists, then lay chipboard floorboards down

i normally use CLS timber to create a stud wall around the walls, fill the gaps of the stud with 50mm PIR if the garage as block / brick piers, i fit the studwork infront of them and fill the void behind it with 50mm PIR, , thats 100mm in total, if space is a premium, i fit the stud work between the piers and just use the 50mm between the CLS stud work, now i know that wouldnt be good enough for building control
staple a vapour barrier to the stud work and then plasterboard

As for the flat roof, you will be able to get plenty of PIR between the roof joists then plaster board, of course if the ceiling is plasterboarded, you will need to rip it down, once again, a vapour barrier and plasterboard

Wall mounted oil filled rads are great for keeping the room warm, low cost to run, looks nice, i have fitted underfloor heating ( warmup is a fairly decent brand ) in the past aswell, but thats a personal choice, maybe use both

I honestly find 50mm PIR warm enough, of course you can go 75 / 80mm ( i have 80mm in the ceiling of my new shed ) I am happy to go with 50mm for the floor and walls, then 80 - 100 for the roof

the big question is how much bending of the rules are you happy with, keep in mind if you do come to sell the house, you will only be able to sell it as a house with a " garage / shed / store room " and most likely have to pay for a £150 Indemnity insurance to cover the building