An encroachment occurs when:

An encroachment occurs when one property extends into another's space. This situation typically arises when a structure, such as a fence, building, or tree, intrudes upon the land of a neighboring property without permission. Encroachments can lead to legal disputes between property owners and can affect property value and use. The concept is important in real estate because it highlights the need for clear property boundaries and understanding of land rights.

The other choices do not accurately define encroachment. While unclear property boundaries can contribute to misunderstandings about encroachments, they are not, in themselves, what constitutes an encroachment. Additionally, the sharing of common boundaries among properties is a normal aspect of real estate ownership and does not imply any unlawful extension or intrusion. Lastly, constructing structures without permits pertains to zoning and building regulations rather than the spatial relationship between properties that defines an encroachment.