we compromised

Me and my young lady came to a compromise, she had me dragging on her ankle looking like a puppie with sad face for a week or two.so instead of the spare room i was told if it is in garden i could have fish room. so at this stage i have a 12' x 8' fishroom , i designed it so if i wanted to make larger to 12' x 16' could convert. electician mate did me a favor and put all the electrics in. the walls,floor and roof, have 4 inches of insulation  .    that is as far as i have got ,so no internal design done yet.         so happy boy

Sounds like an excellent compromise.
Whilst it might be tempting to cram it full of tanks, I'd strongly recommend including a sink and water supply that's buried sufficiently deep that it doesn't freeze in winter. Especially as you have the option of expanding once the first one's full.
We'd love to see photos of your progress.

water supply that is the big question in my head at the minute run a permanent water supply or semi permanent. is it a dripper system would need permanent supply or do outside tap with no check valve inside it , so i can reverse the flow. i will stick hose pipe on the outside of fishroom when i need water.

You lucky thing! I'm very jealous! Good luck with it and yes, would love to see photo's too!