Legal Question in Construction Law in Illinois

Insufficient info. What do you mean "higher"? Are you in a condo? Then this may be an association issue with a shared waste stack. If it's an apartment, your landlord may be liable. But since you have no problem the blockage may be in their waste line before it ties into the shared one. This may require a plumber to investigate and clear. If you are in a condo your downstairs neighbor should contact the association (or landlord in a rental building) first.