Don't forget you can also listen to beautifully recorded reference CD's in your environment and get used to how they sound.  In a way you can eliminate the room from the picture all together.

Kalle Rantaaho
Thanks for your input, friends.
 
That  Bitflipper link by Gustabo and EQWizard will be the routes I'll go. ARC is not in the picture.
And as there's practically no room for further room treatment, I'll just have to know the weaknesses and then live with them.

Just so you know, I used that link provided to treat my room and then I used ARC to finish/clean-up what I couldn't accomplish with treatment.
I swear by ARC, it works and it works extremely well!
I think it's on sale right now too!

Kalle Rantaaho
There's actually not much point in describing the room, as there are no alternatives to the positioning of things. I'd just want to find out how good or bad it is the way it is now. If it's bad, I'll just have to live with it.

 
Maybe it's best not to know .
 
I wouldn't let anything discourage you from making music, just be aware that if you create a piece that sounds great in your room, it may not sound good anywhere else.  Concrete is a great reflector and your ceiling is pretty low.
 

The room in total is 7m x 5m but it's split at 3,5 m to separate the other half for storage room.

 
The following is just an example of the fun you can have.  I don't know how accurate your measurements are, but I entered them into Ethan Winer' Mode Calc app and here's what came out :
 

 

 
These are (some of) the frequencies that may give you issues as they are ones at which your dimensions will cause standing waves.  Ideally, they're as spread out as possible.  In your case, for example, around 250hz you have three modes close together, so you can expect oddities around there.
 
You can sweep a sine wave at different frequencies while moving around the room to give you some idea of what's going on (that's more or less what Room EQ Wizard does). 
 

The closet-wall splitting the rooom is filled with clothes in hangers.

 
Can you open it towards you ?  Full width of the room ?  That can certainly help.
 

 
As mentioned, it sounds surprisingly decent, but I'm interested in mapping the less obvious weaknesses.

The only thing that would be practical to do is put some absorption at the primary reflection points on the side walls (and ceiling but you don't really have room for that).  Have someone hold a mirror on the side wall and when you can see your monitor in the mirror, that's where to put the absoption.  Absorption panels on legs or wheels (gobos) are versatile.
 
What kind of music are you playing and at what volume ?  It's good that your room isn't square or worse cubic.  It would be interesting to see some graphs from REQW just for fun if you're inclined to spend some time learning how to use it, then measuring at different locations.  Probably not worth the trouble unless you're really interested.
 

Thanks for taking the time, Paul.
 
I'll be back when I have studied the EQWizard and tried some measurings. It seems it really takes some
studying to get something out of it. I'll have some free time around easter, so maybe then.
 
I think my saver very likely is the storage half of the room, which is capable of absorbing  a lot :o)
In the music half the only place to put panels according to measurements/needs is the ceiling. And it's almost
fully panelled already.
 
I've been hobby-recording for over twenty years, so the mix translation problems and getting aquainted with different rooms
are familiar to me. I mostly do mainstream rock.

I've used REW5 extensively.  Click on Generator, set that to 40, walk around your room and you can hear where all the bass frequencies are piling up.  The worst place is probably where you'd want to construct and place a bass trap or 2.
 
To shoot your room you need a calibrated mic, like the SPL meter from Radio Shack.  I managed to get my listening position flat to +/- 5db, which I am told is "good".  To improve that would require doing more stuff, like hanging a 4ft by 6ft panel stuffed with 2" of Roxul SafeNSound from the ceiling over my listening position ... which I am not doing, because I stupidly used REW5 to give myself a hearing test ... and failed.   

As mentioned, the only place left for panels is the ceiling (assuming they're thin :o). No room for bass traps or anything of similar size. I just hope the solutions with shelves and sofas and the storage half help me out.
No purchases in sight, so I'll manage with what I've got. I'll let you know when I know something.
 

I JUST FINISHED...
relocating all of my studio equipment....
had a minor flood, had to move stuff anyway,
now that THAT is taken care of, took the opportunity to make a new layout of my studio space.
moved my mixing area, centered on a wall,
whereas before it was in a corner..
this made a BIG difference.
 
moved my treatments around,
mostly by ear and line of sight, common sense stuff..
with my newer small nearfields (JBL 305's), i went with a slightly wider triangle (48" center to center)
and in my space, this seems perfect.
nice wide sweet spot.
 
 
i used the IK Multimedia ARC 2 software,
at about 80db,
i dialed in 14 test points and the curve was not nearly as intense as it was previously,
obviously moving my gear around and really getting things centered,
made a big improvement.
 
very close to flat with the ARC2 plugin,
i checked my older mixes that i had been happy with,
and they ALL needed minor tweaks,
in both the low end and high end.
 
after a couple of hours of comparing level-balanced pro mixes with my best self mixes,
i'm happy with the outcome.

this move, gave me a chance to setup a new guitar recording layout as well....
i've decided, that my old Roland 1x12 cab, that i have opened up in the back, and that i have placed my best driver in (Celestion Heritage G12H-55), sounds best facing the wall and miced from behind....
a really rich sound, i'll be experimenting a lot with this shortly.....
my newer AVATAR closed back 1x12, with an older rare driver in it (late 70's Celestion G12C-30) sounds best miced from the front.

so those two, plus my Demeter Silent Speaker Isolation cabinet, comprises my current guitar setup.
i have a 2nd Roland closed back cab, with a Carvin British series driver in it, that is not being used currently.
 
 
basically,
i have cheap stuff,
but make the most with it.
 

bitflipper
Room EQ Wizard is the place to start - not ARC.
 

Hi bit, 
 
Thanks for the tip on REW. Would this SPL meter work as well as the CM140? It is newer but measures 125hz to 8K. 
 
http://www.amazon.com/Galaxy-Audio-CM130-Sound-Level/dp/B0002GWFG4/ref=sr_1_2?ie=UTF8&qid=1424719616&sr=8-2&keywords=galaxy+cm-140
 
I could use an SM57 through a preamp if needed.  I am interested in this REW system before I consider splurging for ARC 2.
 
 

Ok. I did a quick starter test.
I downloaded a high quality white noise wav from here
 
http://www.audiocheck.net/testtones_whitenoise.php
 
I imported it in a SONAR project. 
Then I played the noise through my monitor speakers and recorded it with AKG c3000b microphone at my listening/mixing spot.
 
Analyzing with Voxengo SPAN the curve has a clear dip at 100-150 Hz and 1000-1500 Hz, the latter of which is already visible in the manufacturers frequency chart, so it depends (mostly) on the microphone. Otherwise the curve is not straight, but forms an "evenly wavy" horizontal line. 
If that is reliable info, I assume the only thing to do is use an EQ to correct that when mixing (and trying to remember to switch it off when exporting :o).
 
I'll do some more elaborate measurings when I understand enough about Room EQ Wizard. I'm a little unsure, though, if it's worth the trouble without a proper measuring microphone.