The Fourth Amendment forbids unreasonable searches and seizures. The Supreme Court has said this generally requires a warrant to enter a home, with some exceptions.
It has long been established, as a general rule, that police may enter a home without a warrant if an occupant of the home with full authority over it invites them in. As a matter of property law and generally understood social convention, of course you can go in if one roommate invites you. You don't have to go around and get consent from every roommate.
This cohabitant rule might be considered an application of the general rule actually in the Constitution. Such an entry is not unreasonable. It might be considered an exception to the court-created warrant requirement. Either way, it is well established.
Eight years ago, in Georgia
, the Supreme Court made an exception. Randolph, present at the time, objected to the entry of the police into his house, but his wife "readily gave" consent. This was held to be a violation of the Fourth Amendment in a 5-4 decision. "The question here is whether such an evidentiary seizure is likewise lawful with the permission of one occupant when the other, who later seeks to suppress the evidence, is present at the scene
and expressly refuses to consent."
What if the objector is not present? That was the question in the case decided yesterday, Fernandez v. California