If the accessory dwelling is “legal” (has separate address, own mailbox and own utility meters), then it would enhance the value of the property, even if situated on the same lot as the main house. This “enhancement” to the appraisal would likely be based upon the amount of monthly rent it could fetch. If it’s just an unpermitted MIL unit which uses the same address and utilities from the main house, then the main house description would just include the BR’s/BA’s of the accessory unit in its own description and the accessory dwelling may or may not add value to the property, depending on the size of the lot and how it is used.