There's a knock on the door and someone from the police says they want to come in. What are your rights and what can the police lawfully do?
Broadly put the police can enter your home if they have:
a warrant that allows them to enter your home to arrest someone,
a search warrant, or
permission from you or from someone else in authority in your home.
They can also enter your home in some urgent situations.
It is important to remember that the police are there to make sure people are protected and justice is done. Cooperating with the police is a good thing. That said, your house is a private place and unless you want the police to come in and look around you are totally entitled to refuse entry. Absent a warrant to enter (or urgency as described below) the police have no more right to enter your home that anyone else. They need permission to enter. This permission can come from you, or from someone else in your home who has the authority to permit them to enter. This person should be an adult and capable of understanding they are giving permission – someone who is highly intoxicated may not be able to give permission to enter. The person giving permission to enter must be able to understand the nature of the police conduct to which he or she was being asked to consent. While there is a distinction between allowing entry and allowing a search it is best to refuse entry if you want to avoid a search.
If you do not want the police to enter your home, tell them. In the unlikely event the police enter your home without permission, do not try to stop them. Tell them again you are not allowing entry and you want to call a lawyer.
If the police have a warrant to enter you must let them in even if you don't want them to. A warrant is a written order from a judge or justice of the peace that gives the police the right to search your home and arrest people hiding there or to take certain items that they find. If the police have a warrant, they can use "reasonable force" to enter your home. If you try to stop a legal search, you can be charged with obstructing the police.
The police should show you the warrant. If they do not show it to you, ask to see it. If you see errors in the warrant – the wrong address or perhaps the time for the search has come and gone or the warrant is not signed – tell the police and say you are objecting to the entry. That said, do not in anyway obstruct the police. Politely but clearly state your objection and then stand back and allow the police to search.
Finally, the police can enter without a warrant or permission when they have reasonable grounds to believe that they need to enter to prevent someone inside from being seriously injured or killed. These extreme circumstances are rare but reflect a reasonable exception to the rule requiring permission or warrant – if the police hear a scream and a gunshot they can enter a house right away without waiting for permission or a warrant. Similarly, the police can also enter your home without a warrant or permission if they are in "hot pursuit" of someone whom they have the authority to arrest. There are few other extraordinary times when the police are allowed to enter in urgent circumstances, for example, to stop the imminent destruction of evidence of serious crime. Again, these are rare exceptions to the usual rule requiring permission or warrant to enter a home.