Get to Raisa's base (Dying Light)

I tried, but to no avail. Hopefully someone can write how to do this.

I also went in, BUT no further. Stumbled on by chance.
Neither the sewer nor the 2nd floor.
Before "entering" there was a warning - in the sense of the last location = the end of the game.

I climbed up there with the help of a cat. You point the cat on the roof and fly.