it depends.
different states have different laws but it's more or less like this:
an apartment or house shared by unrelated adults have different areas for expectation of privacy.
the 'common areas' are the kitchen, living room, dining area, garage, and any bathroom that's not inside a room. anything in these areas is the responsibility of both residents.
the 'private areas' are the rooms. these areas are only private if they are locked. Say Tom and Craig live together. if the cops have a warrant, they should say for what it is and who's stuff they're going through. so if they have a warrant for the home of Craig, then that's the common areas and Craig's room. If Tom has a lock on his door, and they jimmy it open, and they find tom's 25 fully mature plants, it's inadmissible (no smell was detectable outside), because Tom has an expectation of privacy against being searched.
normally cops will make it very difficult for you to pretend that you didn't know anything. they'll say if you confess then you'll get drug treatment classes. if you try and pretend like you didn't know anything you'll have to go to court, spend tons of $$ on lawyer's fees, etc. etc.
but confessing is stupid.
if you try and pull that "I didn't see those 6 plants in my closet", "I didn't smell that skunk like stench, how should I know what that was?"... you'll be screwed...
if somebody is innocent and facing jail time they'll point the finger towards the guilty when those 'cuffs come out, even if the cops are just bluffing to get more info...... 'nuff said....