Garden Studio

Dear all, This is my first post. Thank you all so much for an invaluable resource! Ok so I'm planning to build a small studio at the bottom of my garden (in South East London. UK). My original plan was to have it built out of blockwork (concrete bricks) I got some quotes and the price was far too much for what I could afford so I had to go to plan B. As well sound engineering I have been a carpenter for a few years. Although I specialise on fitted furniture I have done some building work so I have decided to build it myself (timber structure) hiring some help for some stages. My plan is as follows: - Building size would be 5.5m x 3.5m x 2.6m - Due to cost, ease of installation and environmental impact I have decided that rather than having a concrete slab laid I would like to use a system called Swift Plinth: https://www.swiftfoundations.co.uk/swift-plinth-plus/ -The building's base would be made out of 10x2 treated timber sitting on the swift plinth - My thought was to build the walls using 4x4 timber 600mm centres. The exterior would be covered with a layer 18mm osb board, a layer of 18mm external grade plywood and cladding. - The cavity would be stuffed with insulation. 100mm Rockwool? - The interior would have a layer of plywood and two layers of plasterboard with green glue between layers and caulking all gaps. Finally the interior would be plastered. - The roof/ceiling would have same treatment as the walls. - For the floor I would have a layer of 18mm plywood, a layer 22mm T&G chipboard flooring, A layer of flooring insulation and finally some T&G 18mm engineered flooring. - I would like to have two top level windows (approx 400mm x 1000mm) Each window would have two panes of double glazed 6mm glass (I might sack this idea if it proves impractical and just do with no windows) - I'm planning to have two doors. Each one made gluing 3 x 18mm pieces of plywood and using rubber seals all around My main question here is about isolation. The room would be one open plan room. Generally I monitor at medium level with short loud bursts. I don't plan to record drums. Just acoustic instruments and guitar amps. For these I would build an enclosure so they don't make too much noise. My nearest neighbour is 25m away. My main questions are: - Does this sound like something viable? In your opinion would I get enough isolation? - Since the base frame would be slightly elevated would it be worth stuffing the cavities of the frame to stop it resonating? Would it be better to just have a concrete base laid? - Do I make any sense? Any help MUCH appreciated. Thanks in advance!! Cesar.