Generally speaking, you do not have to let the police into your home without a warrant. They can certainly ask for your consent, and they often do. But you are not obligated to allow them to come in, even if they try to convince you that you are.
Of course, they can then get a warrant and, if they do, they can come in without your consent. They’ll usually tell you they have the warrant and show it to you, and then you can either allow them in or they’ll just come in regardless. But that warrant is the only thing that gives them permission to act against your wishes, as a judge has stated that there is reason to do so.
There are also a few reasons for a warrantless search, though. Probable cause is needed, and the police have to be able to justify their actions later. If the court decides they didn’t have enough reason for that warrantless search, the evidence obtained during it can be thrown out. A couple of reasons they may use include:
They can see something suspicious in plain view
If the police knock on your door and spot something they deem suspicious, it may be enough for a more extensive search. For instance, if they’re investigating a shooting and you have a firearm on the kitchen table or if they’re investigating drug sales and there are apparently illegal drugs on a counter behind you. If you’re worried that they may invent an excuse while talking to you, one potential solution is to only converse with them outside.
There seems to be an emergency
If the police believe there is an emergency, they can come inside. One common example of this is if they hear shots fired inside a home. They do not have to wait to get a warrant when they know someone may be in danger. Other reasons could be hearing shouts for help or seeing that a building is on fire. These exigent circumstances have to be serious, but they can be used.
If you do end up getting arrested, then you need to be well aware of your legal options and your rights.