It´s a recreation room.
You can read this in a Wikipedia about "The Simpsons"
"
The rumpus room has only been seen in a few episodes. 
It is seen in "Three Men and a Comic Book", "Separate Vocations", and "Lady Bouvier's Lover" with the same distribution (with cabinets beside the window). 
In "Radio Bart" it is seen, but…

See these pictures and think again about the 45 degree angles:

It´s a recreation room.
You can read this in a Wikipedia about "The Simpsons"
"
The rumpus room has only been seen in a few episodes. 
It is seen in "Three Men and a Comic Book", "Separate Vocations", and "Lady Bouvier's Lover" with the same distribution (with cabinets beside the window). 
In "Radio Bart" it is seen, but…

The entire first floor of the home is built to scale in a studio but the first floorplan sceens were filmed in removable sets.
When Lorelai's bedroom appears for the first time in the episode 1X02 apparently the first fllorplan only contains her bedroom at one side (above the living room) and a bathroom in the other…

Yes, It´s a recreation room.
You can read this in a Wikipedia about "The Simpsons"
"
The rumpus room has only been seen in a few episodes. 
It is seen in "Three Men and a Comic Book", "Separate Vocations", and "Lady Bouvier's Lover" with the same distribution (with cabinets beside the window). 
In "Radio Bart" it is seen,…