California Law

California Penal Code section 693 provides that:

Resistance sufficient to prevent the offense may be made by the party about to be injured:

1. To prevent an offense against his person, or his family, or some member thereof.

2. To prevent an illegal attempt by force to take or injure property in his lawful possession.

It should be read together with sections 692 and 694.

Judicial Council of California Criminal Jury Instruction #505 reads:

The defendant acted in lawful (self-defense/ [or] defense of another) if:

1. The defendant reasonably believed that (he/she/ [or] someone else/ [or] ) was in imminent danger of being killed or suffering great bodily injury [or was in imminent danger of being (raped/maimed/robbed/ )];
2. The defendant reasonably believed that the immediate use of deadly force was necessary to defend against that danger;
   AND
3. The defendant used no more force than was reasonably necessary to defend against that danger.

Belief in future harm is not sufficient, no matter how great or how likely the harm is believed to be. The defendant must have believed there was imminent danger of death or great bodily injury to (himself/herself/ [or] someone else). Defendant’s belief must have been reasonable and (he/she) must have acted only because of that belief. The defendant is only entitled to use that amount of force that a reasonable person would believe is necessary in the same situation.

If the defendant used more force than was reasonable, the [attempted] killing was not justified

According to "California Self-Defense Laws" (a page posted by a CA law firm:

California law allows use of force in self-defense or defense of others when you reasonably believe that you or they are in imminent danger of physical harm, and that force is necessary to stop the danger. However, you may only use the degree of force reasonably necessary under the circumstances.

The page "California Self Defense Laws" from Findlaw reads:

The right to use force -- even deadly force if necessary -- to defend oneself is a broadly accepted principle of the criminal justice system. But while all states allow defendants to claim self defense if they can back up such claims, states differ on the scope of what may be considered appropriate use of force and when it may be applied. ... As a general rule of thumb, any force used against an intruder must be proportionate to the harm reasonably feared.

In People v. King {Crim. No. 20380. Supreme Court of California. August 29, 1978.} The California Supreme Court held that self-defense was a possible defense to a charge of possession of a handgun by a felon, and wrote about the right of self-defense in CA law. The opinion reads:

[I]t is apparent that the conviction cannot stand if the trial court erred in refusing to instruct the jury regarding the right of self-defense as it related to the section 12021 charge. ...

When enacting section 12021 and its predecessor statute, the Legislature is presumed to have been aware of the several existing statutes giving any person the right to use force, including deadly force in appropriate circumstances, in defense of self or others. This right has been included in provisions of the Penal Code since its enactment in 1872. Section 692 provides:

Lawful resistance to the commission of a public offense may be made:

1. By the party about to be injured;

2. By other parties."

Section 693 provides:

Resistance sufficient to prevent the offense may be made by the party about to be injured:

1. To prevent an offense against his person, or his family, or some member thereof.

2. To prevent an illegal attempt by force to take or injure property in his lawful possession.

Section 694 extends the right to defend others, providing:

Any other person, in aid or defense of the person about to be injured, may make resistance sufficient to prevent the offense.

Although the extent of these rights has been defined and circumscribed by judicial decision (see, e.g., People v. Ceballos, supra, 12 Cal.3d 470) none of these sections has been amended to restrict the rights affirmed therein since its adoption over a century ago.

Civil Code section 50, affirms the same rights, providing:

Any necessary force may be used to protect from wrongful injury the person or property of oneself, or of a wife, husband, child, parent, or other relative, or member of one's family, or of a ward, servant, master, or guest. It, too, was adopted in 1872, and it was amended in 1874 to expand the right of defense first declared to encompass guests. Similarly the provisions of sections 197 and 198 governing the circumstances in which homicide is justifiable were included in the Penal Code of 1872, and are traceable to the Statutes of 1850.

...

Use of a concealable firearm in self-defense is neither a crime nor an unlawful purpose. ... We conclude, therefore, that the prohibition of section 12021 was not intended to affect a felon's right to use a concealable firearm in self-defense, but was intended only to prohibit members of the affected classes from arming themselves with concealable firearms or having such weapons in their custody or control in circumstances other than those in which the right to use deadly force in self-defense exists or reasonably appears to exist. Thus, when a member of one of the affected classes is in imminent peril of great bodily harm or reasonably believes himself or others to be in such danger, and without preconceived design on his part a firearm is made available to him, his temporary possession of that weapon for a period no longer than that in which the necessity or apparent necessity to use it in self-defense continues, does not violate section 12021. As in all cases in which deadly force is used or threatened in self-defense, however, the use of the firearm must be reasonable under the circumstances and may be resorted to only if no other alternative means of avoiding the danger are available. In the case of a felon defending himself alone, such alternatives may include retreat where other persons would not be required to do so.

As the answer by Greendrake says, if you have a a legal reason, to enter property, you re not a trespasser. Tresspassing is generally defined as entering on property against the expressed will of the owner or lawful occupant, without lawful authority. A firefighter is not trespassing when responding to a fire, even if the owner orders the firegighter to lave. A buildign inspectort is not tresspassign while making an inspection. Neitht is a police officer with a warrnt for entry, or a person with a court order to recover property.

In some cases such lawful authority must be communicated to the owner, in others it need not be, this depends on the nature of the authority and the exact provisions of the governing law  .

Whether such a person entering with authority may be armed depends very much on the local law, and the nature of the authority. In some cases and some places such a person is free to be armed, in others this would be illegal.

If one is lawfully present somewhere and is attacked. one is entitled to defend oneself (or another), although there may be a duty to avoid a confrontation by retreating if this is reasonably possible when not in one's own home in some jurisdictions (such as California). But the use of deadly force (such as a gun) is generally not lawful unless there is an imminent threat of death or serious injury. None of this is legally different when one is lawfully on another's property than when one is in a public place.

As the answer by Greendrake says, if you have a a legal reason, to enter property, you re not a trespasser. Trespassing is generally defined as entering on property against the expressed will of the owner or lawful occupant, without lawful authority. A firefighter is not trespassing when responding to a fire, even if the owner orders the firefighter to leave. A building inspector is not tresspassing while making an inspection (if local law grants an inspector access). Neither is a police officer with a warrant for entry, or a person with a court order to recover property.

In some cases such lawful authority must be communicated to the owner, in others it need not be, this depends on the nature of the authority and the exact provisions of the governing law.

Whether such a person entering with authority may be armed depends very much on the local law, and the nature of the authority. In some cases and some places such a person is free to be armed, in others this would be illegal.

If one is lawfully present somewhere and is attacked, one is entitled to defend oneself (or another), although there may be a duty to avoid a confrontation by retreating if this is reasonably possible when not in one's own home in some jurisdictions (such as California). But the use of deadly force (such as a gun) is generally not lawful unless there is an imminent threat of death or serious injury. None of this is legally different when one is lawfully on another's property than when one is in a public place.