I am using three different locations at the moment:
Fincan Neukölln, Altenbrakerstr. 26 (corner Nogatstraße),
12051 Berlin, https://www.fincan.eu
A super sweet, cosy cafe and multi-use space with lots of classes and events, non-profit association held up by volontary work. Go and check them out, eat their cake and food and enjoy some yoga, feldenkrais, pilates, meditation or dance classes.
Theres one stair at the entrance and one inside on the way to the restrooms. Theres a ramp which can be used for this. Unfortuately the restrooms are not wheelchair-friendly equipped, only the size works with a wheelchair.
Aquariana, Praxis- und Seminarzentrum in Kreuzberg, 
Am Tempelhofer Berg 7D, 10965 Berlin,
https://www.aquariana.de
Kristina Messerschmidt designed and established this center in 1998 and runs it ever since. What a powerful Women! And so docile and warmhearted at the same time. You find a huge program of therapy, treatments, and councelling as well as seminars, courses and lectures in this center. And flexible possibilities to rent space, especially for new self-emplyoed people. Have a look! (The picture doesnt show the room I use. The one I use is smaller and has a wooden floor.)
Theres no stairs to get into the building or inside the space, theres an elevator to the 4th floor. Unfortuately the restrooms are not wheelchair-accessable. We could assist you, if that would work for you.
Oyoun Kultur NeuDenken in Neukölln, 
Lucy-Lameck-Str. 32, 12049 Berlin,
https://oyoun.de
The one and only Oyoun, the non-profit culture center at Hermannpltz, that visions, develops and implements artistic-cultural projects through a decolonial, queer*feminist and migrant perspective. I am so happy and grateful to be able to do workshops here. Thanks a million!
The Building and space are wheelchair-accessable and theres a wheelchair-accessable toilet on the same floor.
(The picture does not show the room itself, that one has pillows and coaches as well.)