Source: Warhammer Fantasy: 6th Edition

Moving Into and Around Buildings
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It is not possible for units of troops to enter buildings, courtyards, or small walled fortifications in a conventional formation. In order to do so they automatically break into a skirmish formation (see the Skirmishers section). Although only specified types of troops can skirmish on the battlefield, any troops can skirmish in and around buildings.

Next - Entering a Building