No it is not my design.
The room was designed by a company that specialises in acoustic design.
The room acoustics was was first designed in a digital domain and then improved upon with the real world measurements taken during all stages of construction work.

The sand stone blocks you see on the walls are a vital part of acoustic design. Their effect on the room acoustic was quite dramatic. When we have put them on the walls - and that was the firs thing done to the unfinished room, even before the floor was laid - almost all reverberation was gone. If you ever been in an empty room, you know how bad it is.
We have taken this idea from one of their previous projects - namely a recording studio.
The 3 cylinders you see on the on the wall are acoustic panels made ny german company ACS. They diffuse the sound and also absorb at certain frequencies. I have 3 such a cylinders on left wall, 3 on the rignt, and 5 on the rear wall as well, to diffuse the sound refected from that wall.
The ceiling is unfinished as of yet. We were to cover it partly with suede, but after measuring the room RT-60 (reverberation times) we dropped the idea since that would make the room overdamped in the HF.
I still have to put 5 wooden diffusors there, just over the listeners head, according to this drawing:
The semi-circle you see in front on that drawing is the place where ceiling lowers by 10cm - it is also a part of acoustic design.
The room is also soundproof (a second importand feature) and among other things features soundproof windows, double doors and 11" thick double walls. The measured noise level inside the room is 26dB during the day (measured during rush hours, at 16:00).
As for costs - I honestly do not know. I was finishing the whole flat at the same time, and the costs of my dedicated listening room just got buried in total costs.
If there would be enough interest in my room design, I may open another thread, since I have 100+ pics fromm all stages of construction work.