That would depend on the type of outbuilding. We follow FNMA’s guidance on this topic:

Type of OutbuildingAcceptability
Minimal outbuildings, such as small barns or stables, that are of relatively insignificant value in relation to the total appraised value of the subject property.The appraiser must demonstrate through the use of comparable sales with similar amenities that the improvements are typical of other residential properties in the subject area for which an active, viable residential market exists.
An atypical minimal outbuilding.The property is acceptable provided the appraiser’s analysis reflects little or no contributory value for it.
Significant outbuildings, such as silos, large barns, storage areas, or facilities for farm-type animals.The presence of the outbuildings may indicate that the property is agricultural in nature. The lender must determine whether the property is residential in nature, regardless of whether the appraiser assigns value to the outbuildings