D purchased a lot and subsequently built a house, planted a tree, and built a garage/workshop separate from the house. Which term describes this process?

The process described in the question is best characterized by the term accession. Accession refers to the legal principle that allows a property owner to claim ownership of additional materials or improvements that have been added to their property. When D built a house, planted a tree, and constructed a garage/workshop on the lot, these actions resulted in the transformation of the property by adding new structures and enhancements, which are now part of the land.

This principle underscores that any new developments or improvements made to the land contribute to the overall value and ownership rights of the property. It reflects not just a physical change but also a legal confirmation of ownership over the additions made.

In contrast, the other terms do not accurately reflect the process of adding improvements to the property. Hypothecation relates to utilizing property as collateral without transferring title; severance involves separating a part of the property from the whole; and encroachment pertains to a situation where a structure intrudes upon another property.