Yeah, I measured the dB levels with an application on my computer, had no idea that the results are totally useless.
So why peoples build this useless programs at all ???
The program is probably OK... but you need to calibrate it first! The computer just receives a digital signal from the mic and interface, but it has no way of knowing what that signal represents in the real world. You have to tell it that the level it is seeing is actually XX.X decibels, and from then on it knows how to calculate all other levels (provided that you don't change any settings, or use a different mic!).
Just realized that we almost neighbors, how small is the world
Así es, vecino!
I am feeling, that a double brick wall is the way to go for me, gives me good insulation and materials are easy available, affordable
Right. It seems like it will be your best option.
Must source out Rock-wool sheets, but I guess the heavier and tighter, the better.
Nope! For mineral wool, you need to find stuff that has a density of about 50 kg/m3, roughly. That's the optimum, for most acoustic applications. If you use stuff that is heavier than that, then it won't do so well at absorbing low frequencies. And if you use stuff that is much lighter than that, then it won't do so well at absorbing high frequencies. But probably anything in the range 40 kg/m3 to about 60 kg/m3 would be reasonably good.
Glass-wool is readily available here everywhere and cheap, maybe I use that
If you use fiberglass, then the optimum density is about 30 kg/m3, and you could use anything in the range 20 kg/m3 to about 40 kg/m3. The properties of different types of insinuation are not the same: it is important to use the correct density of each type.
and when I pack it tight together it could work too ???
Nope! You should NEVER compress insulation much, as that changes it's characteristics: it makes it more dense, so it becomes worse for low frequencies. It also prevents the tiny fibers from moving as they should, so it is less efficient at absorbing sound. Just leave it as it is.
Do I really have to flock out 100 Bucks to measure the exact DB levels ?
You will need that meter for many tests along the way, and to calibrate REW when the time comes to do room tuning, and to calibrate your speakers when you do the final setup, and to make sure that you are always mixing at sensible levels, always, for ever....
So yes, you need a meter. And 100 bucks is only a very small fraction of what it costs to build a studio, so you won't even notice spending that little!
I just build a bit thicker walls to be sure NO sound enters into my room
Nope! Isolation does not necessarily depend on thickness, and increasing the mass of a single-leaf wall does not have much effect on the total isolation.... It's a lot more complicated than that, and is not intuitive...
I am thinking of building two 20 cm (or maybe 30 cm for the outside wall) thick walls with a 10 cm gap (what would be the right distance)
Right distance for what? That's like you asking me "I'm going to get 3 liters, is that enough?" Enough for WHAT? Unless you first now what spectrum you need to isolate, and what level you need to isolate at each frequency, then I can't tell you of 10cm is enough, too much, or not enough. There are equations for calculating all this, but I need the input numbers ....
Maybe two 12cm/12 cm walls would do the job too, I have no idea ?
Neither do I, unless you give me the data I need!
Outside wall sealed with plaster,
Right!
inside wall 10 cm Rock-wool on the wall on a 2" x 6" wooden frame (5 cm gap between wall and insulation),
Why do you want a gap? An MSM isolation wall is more effective when the cavity is completely filled with insulation, and it is touching both leaves, but not compressed very much.
covered with wooden panels (10 mm), to make it a bit cosy.
On the inner walls Absorbers made of MDF and Rockwool and maybe some Diffusers too.
Actually the inner insulation could be used as Absorbers, when covered with cloth, instead of wooden panels.
No, no, and no. You do not ever want to create a 3-leaf wall if you can avoid it. That will make your isolation worse, not better. You can only use diffusers if the room is large enough to allow it. Diffusers have lobing patterns up close: you have to keep them far enough away from your ears that the lobing does not affect what you hear.
Bass-Traps in the corners would not hurt too.
It's not that they "won't hurt": it is that you absolutely MUST have them! It is a small room: it will need many large, deep, bass traps. That goes without saying.
15 cm Concrete slap for the floor, with some kind of carpet on top
No. Carpet is a bad idea in a studio: it does the opposite of what you need, acoustically. Forget carpet.
Doors made of double walled Steel, heavy insulated with 10 cm Rock-wool
No. Never use hollow-core doors for a studio: the cavity inside is a resonant system. Use only solid-core doors, that are very, very massive.
1 (2) Small window(s), about 80cm x 30 cm, (just to let some light in) with 10mm/8mm double glass in each wall.
No. Double glazing panels have very lousy low frequency isolation, and having two double panels means you have a 4-leaf system, which has terrible low frequency isolation! What you need is thick laminated glass. One pane of 22mm laminated glass in the outer leaf, and another one in the inner leaf. That's all. And make sure you get laminated glass that has the special acoustic PVB: it is a bit better than that standard PVB.
Ventilation with quiet solar driven computer fans, mounted outside the building, air goes trough trough foam insulated sound traps.
No, no and no! HVAC for studios is VERY complicated: you can't do it the way you suggest. You need silencer boxes, to stop the sound going in and out while allowing the air to flow, they need to be the correct size for your room, you need proper HVAC fans that move the right volume of air at the right speed when faced with the static pressure created by your system: computer fans are not meant to be used with any static pressure load at all! They would simply stall and do nothing. You also need the correct size ducts and registers, and you need a mini-split system to heat, cool, and dehumidify the air.
Yes, you do NEED all of that: you cannot do a studio without it.
This construction would be a bunker, that's for sure and probably cost me a small fortune
Right!
If it gets too costly, I postpone it for next year, no point to safe money and then I have the same what I have now
It will take many months to design the studio correctly: If you hire a studio designer to do it, it will take two or three months, probably. If you do it yourself, it will take MUCH longer, as you will first need to learn all about acoustics (at least 4 to 6 months), then you will need to learn all about studio construction techniques (another 3 or 4 months), then another 4 or 5 months to do the actual design. So yes, I'd agree that you should plan to start building in 2017, if you want to design it yourself. Or perhaps in May or June, if you hire a designer to do it for you.
As there should be no parallel wall on the whole construction,
Why????
That is a myth. Many people think that the walls in a studio HAVE to be non-parallel, but that simply is not true. It is very possible to build a studio that has a rectangular shape, with parallel walls... if it is design properly, and treated properly!
Would the so often recommended diamond shape bring big huge benefits ?
No, none at all: that's another myth...
What about a PYRAMID shape,
- Stuart -
In about a hundred years from now, I should have enough! 
(surprisingly we are still married although this booth could put an end to that).