The Third Amendment states that : "No Soldier shall, in time of peace be quartered in any house, without the consent of the Owner, nor in time of war, but in a manner to be prescribed by law."
This Amendment is probably less important now than it was when British troops were forcibly quartered in colonists homes but it is still a foundation of privacy cases such as Griswold v. Connecticut with the general principle being that your home should be free of intrusion from state agents.