Legal Question in Real Estate Law in California

That would depend on the form of the permission, how it was documented, and exactly what it said. If it is not in writing, you're probably out of luck.

If you built a structure on someone else's land with nothing more than oral permission, or even permission in writing but without giving a valuable consideration, your permission would be regarded as a license - and not an easement, lease or anything more durable. Licenses are revocable at will. Therefore, the owner of the land can probably revoke the permission he/she gave you. Furthermore, improvements such as buildings, fences, etc. attached to the land of another become part of the realty and therefore belong to the owner when the license is revoked or the lease expires. As Mr. McCormick suggests, "your" building has probably become a gift to the land owner. So, the probably-correct answers to your questions are that: wrong, you don't own the building; and Yes, the landowner can deny you access; and Yes, the land owner can break into it without your permission.

You could have maintained legal control by acquiring an enduring right to build and keep this structure on the other person's real property, such as an easement or long-term lease. As it is, the "permission" given is probably revocable, and under the law of fixtures, the structure has become part of the realty.

There are possible exceptions that would give you a better result, including, perhaps, an equitable easement........but you'd need to consult a local lawyer with considerable expertise in evaluating easement problems to see if the proper combination of circumstances is present to merit a court challenge to the land owner's apparent rights.

It sounds as though you are an unlicensed contractor. If you were a licensed contractor, and built a structure on someone else's property, you would be entitled to record a mechanic's lien and foreclose on the mechanic's lien when you were not paid by the owner. In that situation, you would then own the structure.

If you are an unlicensed contractor, and a license was required, you cannot utilize the mechanic's lien laws to protect you, and you do not and will not own the structure.