Snow inside my Sims house because of the way I build it?

Can't see the roof but it likely has a gap somewhere.

Can´t find it 😉 

It seems the snow is gone now. See if it returns next winter season.

After I´ve build a room (red) on top of the small gateway (bevor there was only one wall for the Zinn, yellow), like the big one just on the top, it didn´t change. But building a roof on top of the small gateway works. Than I don´t understand how the big flat roof work?

The flat roof is actually a room with concrete flooring. There is applied a low fence to it instead of ordinary walls.

Thx, as written I´ve build room too (red lines), same way as the flat roof, except the walls right and behind they stayed as normal walls, the others wehre fences and it didn´t work.
What closes the flat roof for the snow, the fence around, the concrete floor?

I really can't say. I would experiment with different solutions. Such as a half gabled roof.

@JochenWeiss It likely is due to how you built the area.  If the game didn't consider it an enclosed room, it would treat it as an outdoor area.  If you use fencing or half walls instead of walls, this can happen, or if a part of the ceiling is missing.  If you place a flat above the area (as I think you wound up doing), and it corrects the issue, then the ceiling was likely the culprit.  If you are still having issues with the build, I could always take a look if you are willing to upload it to the Gallery - you could just call it "Test Snow in House" or the like, and tag me here to let me know it's up.

It´s up, I´ve rebuild it like it was with the bug, not sure if it´s happen again due to it´s Spring already 😉