There are four main circumstances in which a warrant is not required for police to search your house:
1.
Consent. If the person who is in control of the property consents to the search without being coerced or tricked into doing so, a search without a warrant is valid. Note that police do not have to tell you that you have the right to refuse a search, but you do. Also, note that if you have a roommate, he or she can consent to a search of the common areas of your dwelling (kitchen, living room), but not to your private areas (bedroom, for instance). On the other hand, the Supreme Court recently ruled that one spouse cannot consent to the search of a house on behalf of the other.
2.
Plain View. If a police officer already has the right to be on your property and sees contraband or evidence of a crime that is clearly visible, that object may be lawfully seized and used as evidence. For example, if the police are in your house on a domestic violence call and see marijuana plants on the windowsill, the plants can be seized as evidence.
3.
Search Incident to Arrest. If you are being arrested in your house, police officers may search for weapons or other accomplices to protect their safety (known as a "protective sweep"), or they may otherwise search to prevent the destruction of evidence.
4.
Exigent Circumstances. This exception refers to emergency situations where the process of getting a valid search warrant could compromise public safety or could lead to a loss of evidence. This encompasses instances of "hot pursuit" in which a suspect is about to escape. A recent California Supreme Court decision ruled that police may enter a DUI suspect's home without a warrant on the basis of the theory that important evidence, namely the suspect's blood alcohol level, may be lost otherwise.
http://www.legalzoom.com/legal-articles/house-search-without-warrant.html
The smell of weed may be probable cause, but that is not good enough because probable cause is up to the judge to decide not a field officer. Even in the situation of a car a cop must first get a dog to verify that the smell is there before they can enter the car if they have no warrant or consent.
