The law of trespassing is complex and not necessarily intuitively obvious. It is possible to be sued under trespassing law for entering your own property. Most trespassing law is common law, or judge-made law, that has been handed down to the United States from British court decisions.
Possession vs. Ownership
Trespassing law protects the right to possession, not ownership. That is why your tenant can sue you for trespassing if you enter land owned by you but rented to her. Of course, you may have a defence to such a claim. As a landlord, you have the right to inspect the property and conduct repairs as long as you provide your tenant with advance notice and as long as these rights are spelt out in the lease agreement.
Trespassing liability is absolute, meaning that you can be sued for trespassing even if your trespassing was unintentional. For example, if you relied on faulty maps provided by the city government to build a structure on land that you erroneously thought was yours, you can be held liable for trespassing, although in this case you may have a claim against the city government for providing faulty maps.
You cannot be held liable for trespassing if your intrusion was "non-volitional." For example, you tripped and fell over your neighbour's fence. Although this principle contrasts with absolute liability, the test for liability is whether you intentionally entered the area, not whether you knew that you had no right to be there.
Express and Implied Invitations
If you order a pizza, you cannot sue the driver for trespassing when he enters your property because he is responding to an invitation. Invitations need not be express, they can be implied. That is why your mail carrier is not trespassing when she enters your yard without your specific permission.
Government Land and Law Enforcement Immunity
Government-owned land and some private property is protected by statute, making penalties for trespassing more severe. For example, there are civil and even criminal penalties for trespassing on the grounds of a nuclear power plant or a military base. To enter your property, law enforcement officers must have a valid search warrant. However, they can enter areas outside the curtilage of your property without a warrant. The curtilage is the enclosed space of ground and buildings immediately surrounding a home. This includes areas close enough to peer into your windows, but might not include a barn in the back yard.
If a landowner fails to remove a trespasser from his property in a timely manner, then ownership of the property could pass to the trespasser in certain circumstances. For example, you enter vacant property owned by someone else under an express or implied claim of right by stating, for example, "I am claiming this property as my own." Or you build a house on the land and the owner knows about your entry and your claim. However, if the owner fails to commence an ejection action against you, then the property can become yours as soon as the statute of limitations for filing an ejection action expires. You will need to file a "quiet title" action to secure your claim.
Normally, civil damages for trespassing are limited to compensating the plaintiff for any actual harm caused. However, even if no harm was caused, the plaintiff in a trespassing action is entitled to nominal damages, which can be as little as 60p.