Just because someone is a member of the police department does not mean you are automatically obligated to let them into your home. They may want to conduct a search if they are investigating you for criminal activity, but there are only certain ways that they can enter a private residence.
The first way is with your consent. An officer may ask if they can come in and take a look around. You get to decide if you will allow them to do so or not. Many people think this is an obligation, but it is important to remember that you always have the right to say no. You do not have to let the officer into your house.
With a search warrant
To override your lack of consent, the police officer has the option to go to a judge, show that there is probable cause for a search and get a search warrant. This will tell them where they are allowed to search, what evidence they are supposed to look for, when they can execute the search warrant and other such details. With a proper warrant, they can come inside even if you do not give them consent to do so.
In an emergency
Where things can get complicated is that there are cases where it is an emergency, and the police have to act quickly, before they can get a search warrant. They may believe that people in your home are in danger, that evidence is being destroyed or that a suspect they were chasing just entered the house. In some cases, if there is a valid emergency, the police can simply enter your home without a warrant or your consent. But they will typically have to justify those actions after the fact to show that it was not an illegal search.
If you believe the police conducted an illegal search, that may invalidate some of the evidence that they found. This could have a major impact on your case, so be sure you know what defense options you have.