Which of the following situations would likely result in an encroachment?

An encroachment occurs when a structure or physical object extends or intrudes onto a neighboring property without permission. In the scenario where a structure is built over the property boundary, this is a clear violation of property lines and represents an encroachment. This can lead to disputes between property owners over the rightful use of land and often necessitates legal action to resolve the issue.

When comparing this with the other options, a fence built within property lines would adhere to the legal boundaries and not infringe upon adjacent properties. A tree that grows too tall might lead to issues, such as shading or roots interfering with neighboring land, but it does not typically constitute an encroachment unless it physically extends onto the adjacent property. A garden planted solely on one's own property is entirely within the rights of the property owner and does not create conflict with neighbors or property boundaries. Thus, the construction of a structure that crosses property lines is the definitive scenario that leads to encroachment.