In the recent decision in Florida v. Jardines, the United States Supreme Court considered whether police could lawfully use a drug sniffing dog to search a the curtilage of a person's home.
In a unanimous decision, the United States Supreme Court held that law enforcement must apply for and obtain a search warrant before they can enter the private property of a person for the purposes of gathering evidence of a crime.
In this particular case, after receiving a tip, the police conducted surveillance of the defendant's home. Officers eventually approached the house with a drug-sniffing canine dog, who sniffed the defendant's porch area and front door. After the canine signaled positive for drugs, the police used that information and applied for a search warrant and eventually arrested the defendant for drug trafficking.
In reaching its decision, the United States Supreme Court reiterated that when government obtains information by physically intruding on persons or their homes, a "search" within the constitutional sense has occurred. In this particular case, search occurred that required the police to first obtain a search warrant supported by probable cause.
The United States Supreme Court grounded its decision, however, based on the actions of the police officers physically intruding on the defendant's property (and not because they had necessarily used a drug-trained dog). Quite simply, because the officers invaded the defendant's property to gather evidence, a search requiring probable cause was required.
If the officers didn't go inside, what are the property boundaries that are protected?
The U.S. Supreme Court regards the area "immediately surround and associated with the home", or "curtilage", as part of the home itself when analyzing any searches for constitutional purposes. While there could always be some grey areas in determining the curtilage of a home, in most cases, the curtilage or boundaries of the home are clearly marked.
Most certainly in this particular cases, the officers' actions in going up to the porch area of the home and just outside the front door was very clearly an intrusion to the home's property and curtilage, and the officers needed to have obtained a warrant before "searching", even with a drug-trained canine.