If you are present when the police want to search your property, you can tell them that you do not consent and they will not be able to search without a warrant. If you say no and they proceed to search your property illegally, any evidence found could potentially be thrown out in court. For the police to obtain a search warrant, they must prove to a judge that there is probable cause to believe that a crime was committed and that there is evidence related to that crime on or in your property. There are various situations where the police do not need a search warrant to investigate private property, such as when evidence is visible in plain sight.