An accessory dwelling unit is a really simple and old idea: having a second small dwelling right on the same grounds (or attached to) your regular single-family house, such as: an apartment over the garage. a tiny house (on a foundation) in the backyard.
ADU Frequently Asked Questions: Link
A secondary dwelling unit is an additional single-family residence that is allowed on a lot where there is an existing legal single-family residence, provided that the lot area of the subject property is at least two times the minimum lot size specified by the land use district for the subject property. A secondary dwelling unit is subject to Land Use Compliance Review and must comply with the single residential design standards and the standards specified for the subject land use district.