Building first ever studio,use existing room or build new 1?

I live in Ireland, and am moving into my new house this summer. I was planning on building my first ever studio out the back, in a small garden, with houses each side within 40 feet. But there is a spare room, 4.27x4.07 metres inside the house. Its small but from what I can make out now, the amount of room in the garden I have, the rules on building, and planning permission, it would nearly work out the same! And alot cheaper. I am planning a simple (hopefully) 2 room studio, live room and control room. Which will be having full bands playing in it, and drums especially as I am a drummer myself. There is one door in and one window so far, but there is also a fire place. I had a rough idea of what I was going to use in my walls for the studio I was planning on building (thanks to this site); Concrete then 3" gap to single stud frame, the gap filled with rockwool, then a layer of 5/8" drywall onto the frame then greenglue then another layer of drywall. The ceiling the same materials in the same order, resting on the newly built walls. and again not touching roof of existing room. The walls would have just been on the existing concrete floor with probably just a laminate flooring on top of the concrete. I am thinking of just doing the exact same thing on the 2 walls with concrete in them, e.g the ones with nothing on oneside. But with the 2 walls inside the house I'm not too sure. I don't know but I'm presuming its just a normal wall, probably drywall then stud frame with a small gap with some form of insulation then drywall. And if so, not too sure about what to do.Same with ceiling but don't have a clue what that consists of. There is space on the fireplace side of the room to build on, but not too much room. But if necessary I could have my control room out there but it would have to be pretty small in wide (back from fireplace). But there is a good bit of room lengthwise. Do I need it or could i get away with fitting my 2 rooms inside the original room? While still getting a good sound!? Soundproofing firepace and window, I don't know what to do! PLEASE HELP

Here is another little drawing of the room. And a proper pic of the room now.