Police can have quite the commanding presence. So much, in fact, that many people think officers have more rights than they actually do. This leads to far too many people just letting police come to their homes and search around, without a legal search warrant.
Police must have a warrant
If you answer a knock at your door and it's the police, don't let them in without a search warrant. You are under no legal obligation to open your home to police unless they go through the process of obtaining a warrant in accordance with the Fourth Amendment to the U.S. Constitution. Police must have probable cause to conduct a search of the home in which you have an expectation of privacy.
If you let them in without that critical piece of paper, you give up that right. Once inside, officers may attempt to conduct a search, and you could have trouble getting them to leave.
Officers may attempt to guilt you into allowing them to search without a warrant. After all, if you had nothing to hide, why would they need a warrant? Because it's your right to require one to search your home, and it makes police tell you why they are there.
Exceptions to the rule
There are very few exceptions to the rules, but one of the most common is exigent circumstances. Police do not need a warrant to enter your home if probable cause exists to believe that one has or is committing a crime. This is also true if there is reason to believe that public safety is at risk and there simply is not time to get a warrant.
The appropriate course of action
If police do not have a search warrant, you may ask that an officer leave a card and return when they have a search warrant. This gives you time to consult with an attorney regarding the situation. If police say they have a warrant, make sure it's a search warrant before letting them in, since it could be an arrest warrant, which is different.
If it is an arrest warrant, the person named in it should go outside the home for an officer to take them into custody. If you allow police into the home to make the arrest, they may be able to search the home. If police do have a valid search warrant, the law requires you to allow them to enter your home in order to conduct the search. However, you still always retain the right to contact an attorney without delay.