Penal Code § 844
CHAPTER 5. Arrest, by Whom and How Made
To make an arrest, a private person, if the offense is a felony, and in all cases a peace officer, may break open the door or window of the house in which the person to be arrested is, or in which they have reasonable grounds for believing the person to be, after having demanded admittance and explained the purpose for which admittance is desired.