Wiktionary
n. A mother-in-law apartment or garden apartment used for in-house employees
Wikipedia
Servants' quarters are those parts of a building, traditionally in a private house, which contain the domestic offices and staff accommodation. From the late 17th century until the early 20th century, they were a common feature in many large houses. Sometimes they are an integral part of a smaller houseāin the basements and attics, especially in a town house, while in larger houses they are often a purpose-built adjacent wing or block. In architectural descriptions and guidebooks of stately homes, the servants' quarters are frequently overlooked, yet they form an important piece of social history, often as interesting as the principal part of the house itself.