No. In most situations the police need either your consent or a search warrant to enter your house. Most police officers will ask your permission to search because it saves them a great deal of hassle in getting a search warrant. The sufficiency of any search warrant can later be tested to see if it was properly obtained. However, if you give consent, any defects in procedure or evidence will probably not help your situation and the search will probably be allowed.
Motor vehicles are a slightly different matter. Usually the police will be able to search your car if you are being arrested. However, this usually does not include stops for speeding or other minor traffic violations.
If you are stopped by the police in your vehicle, you always have the right to say “No, you cannot search my car.” If the police have the legal right to search it, they will do so anyway. If they were wrong, anything they found cannot be used in court. Unfortunately, if you consent to the search, the items found will likely be used against you, even if they had no legal right to search.
Without your consent, private investigators and detectives cannot search your car or home.