27 CFR §40.70

Where the factory consists of a portion of a building, or where portions of buildings are part of the factory, the factory shall be completely separated by walls from adjoining portions of the building. Such walls shall be securely constructed of substantial materials. The appropriate TTB officer may, wherever he finds that the revenue will not be jeopardized, authorize openings and doors in such walls or means of separation other than walls if such means adequately delineate the factory. The factory shall be accessible directly from a street, yard, common passageway, or other common means of entrance.