Warrantless searches are presumed to be unreasonable (Katz v. U.S.), but the U.S. Supreme Court has acknowledged that a warrantless search may still be reasonable under the Fourth Amendment if it falls within the guidelines of one or more of a limited number of exceptions. The standard exceptions include public or officer safety, search incident to arrest, fleeting targets, border search, booking search, inventory, probation and parole search, and consent.

"A search conducted pursuant to a valid consent is constitutionally permissible. One of the specifically established exceptions to the requirements of both a warrant and probable cause is a search that is conducted pursuant to consent. In situations where the police lack probable cause to arrest or search, a search authorized by a valid consent may be the only means of obtaining important and reliable evidence." (Schneckloth v. Bustamonte)

Over the last six decades, the court has established certain guidelines as to who can give consent, what will make consent invalid, and what the proper scope of search is. In chronological order, the following 17 cases set forth general rules for consent searches that control Fourth Amendment analysis and procedure. I've included citations to the cases so you can locate the opinions if you want to read the facts and the court's full discussion of the points made.

• U.S. v. Jeffers (1951) 342 US 48. During a period of paid occupancy, a motel clerk cannot give valid consent to enter or search the guest's room.

• Chapman v. U.S. (1961) 365 US 610. During lawful tenancy, a landlord cannot give valid consent to enter and search the tenant's residence.

• Stoner v. California (1964) 376 US 483. During paid occupancy, a hotel manager cannot give valid consent to enter or search the guest's rooms.

• Schmerber v. California (1966) 384 US 757. Taking a blood sample for alcohol testing from a lawfully arrested DUI suspect pursuant to an "implied consent" statute is permissible, and evidentiary use of the test result does not violate the driver's Fifth Amendment privilege.

• Bumper v. North Carolina (1968) 391 US 543. A claim by officers that "We have a search warrant," or "We'll be back with a search warrant," makes a consequent "consent" involuntary, and invalid. (It would be permissible to say, "If you don't consent, we'll try to get a warrant," since this implies that a refusal of consent will be honored, and that returning with a warrant is not a foregone conclusion.)

• Frazier v. Cupp (1969) 394 US 731. A person with joint authority and control over an item (in this case, a duffel bag) can consent to its search.

• Schneckloth v. Bustamonte (1973) 412 US 218. During a lawful detention (in this case, a traffic stop), officers may ask for consent to search the person and the vehicle, even though there may be no suspicion of criminality.

• U.S. v. Watson (1976) 423 US 411. Valid consent can be obtained from a person in lawful custody.

• U.S. v. Mendenhall (1980) 446 US 544. The prosecution has the burden of proving valid consent by a preponderance of the evidence.

• South Dakota v. Neville (1983) 459 US 553. If a DUI suspect refuses to submit to alcohol testing in spite of the implied-consent law, his refusal can be used against him at trial as consciousness-of-guilt evidence.

• Florida v. Royer (1983) 460 US 491. Consent obtained during an unlawful detention is not valid.

• Illinois v. Rodriguez (1990) 497 US 177. Police may rely on consent given by someone who reasonably appears to have authority over the place to be searched, even if the person does not have actual authority.

• Florida v. Jimeno (1991) 500 US 248. The person who consents to a search can limit the places to be searched and can revoke consent at will. A general consent to search a vehicle includes all compartments and containers.

• Florida v. Bostick (1991) 501 US 429. Police may request consent to search during a consensual encounter, even though they have no suspicion of criminal activity.

• Ohio v. Robinette (1996) 519 US 33. In order to obtain valid consent, it isn't necessary to tell the person he or she has a right to refuse to consent.

• U.S. v. Drayton (2002) 536 US 194. Consent is not considered to be coerced merely because it is requested by armed, uniformed officers.

• Kaupp v. Texas (2003) 538 US 626. Consent is not valid if given in submission to a claim of lawful authority (here, multiple officers roused an adolescent from his bed with flashlights and the words, "We need to go and talk," leaving him no other apparent choice).

Consent as an Independent Source

Sometimes, police rely on two or more exceptions to justify a warrantless search or seizure. If a court rules that the requirements for one particular exception aren't met, the evidence is still admissible if there was another source of the evidence, independent of the improper route. This is the "independent source" doctrine. It's the reason why it's always a good idea to have at least two ways to skin the search-and-seizure cat.

Consent is an excellent independent source. Even if you think your warrantless search is justifiable as incident to arrest, for example, there's no harm in asking for consent before starting to search. Then, if the magistrate at your suppression hearing rules against you on the propriety of searching incident to arrest, your prosecutor can fall back on the consent, and the evidence is saved (also, sometimes the case is saved, and sometimes your assets are saved from a civil liability judgment). A good rule-of-thumb is to seek consent for all non-emergency searches.

Asking the Right Questions

Trying to be polite, many officers ask for consent by saying things like, "Do you mind if I search you? Do you mind if I search your car? Do you mind if I come inside?" These are foolish questions, because of the structure of our language.

"Do you mind?" means "Do you object?" Therefore, a person who answers, "No," is really saying, "No, I don't object." A person who answers, "Yes," is actually saying, "Yes, I do mind, so you may not search." But most criminal defendants are not going to get on the witness stand and brag about their linguistic skills. They're going to say that "No" means "No, you may not search." Your prosecutor will have a hard time selling "No" in the courtroom as consent to search. But you set yourself and the prosecutor up for trouble by building in this ambiguity in your request for consent.

Better: "OK if I search you? OK if I search your car? OK if I come in?" If the person answers, "Yes," we get to sell "Yes" as consent. If the person says, "No," you know the person is not giving consent. No ambiguity. No wiggle room for the defendant and his lawyer to claim a linguistic confusion that negates consent. (And remember, we have the burden of proving valid consent.)

Your Rules May Vary

As always, check with local advisors for any variations in your jurisdiction.

Devallis Rutledge is a former police officer and veteran prosecutor who currently serves as special counsel to the Los Angeles County district attorney. He is the author of 12 books, including "Investigative Constitutional Law."