The basement represents your subconscious, in your thoughts and wishes that you usually don't pay much attention to.
The late grandfather represents past experiences, past events and the like.
Besides, dreaming of a deceased relative may reflect that you may need help to mention something.
Some people have replaced his stairs with coffins, and the stairs represent development and progress.
But as a coffin, it means that constraints are no longer conservative.
That is, the possible development is limited by external factors.
This may have something to do with your work life (if you are a student, that's another matter)