Complex sentence: Mother-in-law apartment; a small apartment in a larger house, usually with a separate entrance, which may or may not have a communicating door to the main house, and usually not a part of the original design.
Split into simpler sentences: Mother-in-law apartment; a small apartment accessory to a larger house. It may or may not have a communicating door to the main house, but virtually always has a separate entrance and is usually not part of the original design.