For a large property, or estate, what is the term for buildings other than the main residence that are separate and unconnected?
Consider outbuilding, dependency, annex, and lodge.
dependency: outbuilding; annex
annex: a building added on to a larger one or an auxiliary building situated near a main one.
lodge: a house on an estate originally for the use of a gamekeeper, caretaker, or porter
Usually called a guest house in the US.
Now there are also several terms depending on what you need. The term detached is used formally. A detached garage. A detached guest house/quarters.
There is also the outhouse for those still with outdoor toilets.
Cabana next to the pool.
Shed that houses your outdoor tools.