Is this "Secret Cabin" supposed to be like this?

Doing the Cult Questline, and clearly this cabin is unfinished. I’m either just unable to find the stairway, or I’m royally screwed.

EDIT: I did do some searching around the house. Tried prying the windows and such.

Try moving,smashing, or disassembling furniture. Sometimes stairs can be underneath instead of somewhere you can see them.

I did. I moved everything. I eventually just decided to dig straight down using mining equipment.