Just a quick note : please excuse my english, don't hesitate to correct me if I use the wrong words, especially in the "building vocabulary", I'm not really used to it..! And most of all, huge thanks for this wealth of information that is john's forum, I never found that level of information on any french website !
EDIT : I edited my post for more clarity, I hope it will be easier for you to come along and follow my project !
So me and my girlfriend recently moved in a new house, that's the good point ! Things is that I can't practice regularly on my drumset, and I think i'll become crazy very soon
So I started planning this project seriously (I read Rod's book, spent countless hours reading posts here, pretty time consuming I must admit !), but now i'm going round in circles ! You're my last hope guys and I have to request your mighty knowledge ! 
MY GOALS
As every fellow drummer, my goals are to have a drum space where I can practice daily without disturbing the neighbours ..! I'll also teach some of my students on the week end, so it has to be quiet enough that the neighbours can still be in their garden without me bothering them ( I guess aroung 45-50dB of noise from the outside would be fine ...). I didn't measured properly the sound of my drums because the actual space where I want to build my booth isn't sealed at all (there's almost no difference if I play doors closed or open), but I think i'm at a quite normal level (meaning : LOUD, around 105-110dB probably). I may record a couple of things here and there, but nothing for "professional use". My budget is more or less 10,000€ for the booth.
STARTING POINT
My first and biggest problem is that I can't really decide between building the drum room in the basement (low ceiling) or in the garden shed. Let's look at the basement first.
1°) THE BASEMENT
Main problem : Height of the ceiling
The house is recent (2008), the dimension of the outter leaf would be 4.33m x 3.8m (can be more or less if needed) x 2.18m high (ouch !).
Existing walls are made up of 20cm "hollow concrete block", I have a 10-15cm thick concrete slab and a 15cm concrete ceiling.
Regarding the isolation, I was thinking about beefing up the outter leaf by screwing 2 sheets of drywall with green glue in between on the existing walls and ceiling. Note that I'll also have to "close" one side of the room as it's an "open basement" (as you may see on the 1st picture). I'm not sure whether I'll build this wall using "Hollow concrete block" so that all the outter leaf will be built in the same material or if I can make a woodframe / fiberglass / drywall type of thing. I must admit that the woodframe / drywall option will be more convenient to build and (eventually) to remove if we move in the future but will it create a weak spot ?
Also, I was thinking about the ceiling height issue. Maybe I could use a suspended ceiling system (like this one but will it be enough to avoid the drum sound to leak in the rest of the house ? (living room is just above).
2°) THE GARDEN SHED
Main problem : Sound isolation (4-5m from the closest neighbour !)
It's an old building (1960), at the moment the dimensions of the outter leaf would be 4.3m x 3.35m x 2.12m tall at one side and 3m10 at the other but I intend to build a flat concrete roof at around 3m height (more on that below).
Existing walls are made up of 20cm "hollow concrete block", a 8-10 cm concrete slab for the floor with 30cm of concrete for the foundation (under the existing walls), and an old fiber cement gradient roof (which will be removed if I go with the flat roof option). The shed is placed side by side with 2 garages (one on the left, one on the back) that may help me for the isolation on those two sides (note that each buildings have it's own separate walls).
As I said, the existing roof will be replaced by a flat roof, probably beams & slab + 5cm minimum (maybe 10 ? I'll have to ask the structural engineer) of concrete poured on top + several layers of waterproof materials. I intend to seal the existing window with hollow concrete blocks (I don't think that putting "full" concrete block would be usefull if I only put it at this place). I'll remove the existing door and replace it by an "acoustic" exterior door after replacing the existing "door frame" by a "beefed up" one. I'm also concerned about the HVAC because I don't want to create flanking path (of course) as well as I don't want it to bother the neighbours by it's own sound (chances are that it may be outside the shed...
).INNER LEAF (I'll complete this section more precisely later on)
Depending on several points (mainly the place of the booth), I'll probably go with a wood frame composed of 2x6 wood studs with 20cm air gap filled with 20cm thick 28kg/m3 fiberglass insulation (just to be sure...is it ok to fill the entire 20cm air gap with fiberglass ?) and probably 3 layers of drywall with 1 or 2 tubes of green glue in between each sheets.
More details to come...
MAIN QUESTIONS
1°) Will it be possible to reach this level of isolation and if yes, which one would be easier to isolate ? The basement with the low ceiling ? (possibly less air gap and less layers of drywalls) Or the garden shed ? (not underground and closer to the neighbours). As everybody else, I'm a bit concerned about the end result.
Especially if I build my booth in the garden shed : is it possible to isolate enough an exterior building (meaning : a "non underground" building) so that neighbours can be in their garden and don't be bothered by the sound of my drums ? When I say is it possible, I mean with my actual budget hehe 2°) For the shed, I contacted several builders for the roof issue. One of them suggested polystyrene slab (like this) instead of concrete slab (like this one . Which one will be better in my case ? He told me that it was an "acoustic" model but on the other hand, I know mass is better...Still, I was wondering if the concrete poured on the top would be enough for the mass and if polystyrene would be a better choice in this case ?..
Another possibility would be to use "metal roof plates" on which I can pour up to 12 cm of concrete. Will this be better in terms of acoustic isolation ?
3°) In France, the average thickness of drywall is more or less 12.5mm. I found 15mm and even 25mm plasterboards but they are either much more expansive or hard to find. My question is : if 12.5mm have more density / weight than a 15mm plasterboard, is it better to use the 12.5mm even if you guys generally recommand 15mm thickness minimum ?
By example : I have found 2.5x1.2 15mm plasterboard that weight 38.10kg (846kg/m²) and costs 11.87€ /m². On the other hand, I have a 2.5x1.2 12.5mm plasterboard that weight 45kg (1200kg/m²) and costs 8.20€ /m².
Which one to pick ? Density or thickness ? Isn't it better to put let's say 3 layers of 12.5mm instead of 2 layers of 15mm given that it's almost the same price and that it would be heavier ? I have read on the forum that thickness is better for low frequencies but I think it was also because the thickness added more mass compared to a thinner plasterboard, which is not my case here. I'm a bit confused sorry
Many thanks in advance for reading me, I hope I was clear enough for you to understand the overall project and be sure that I'll post news whenever I'll have any updates (whether it's construction or planning) !
See you around,
Etienne
Not so good. Especially for drums.
