Ну вот, опять будет теологический спор на десяток страниц =/
А вообще по рульбуку здание принадлежит армии в целом, а не какому-то конкретному детачу.
• At the start of the game, all buildings that were taken as part of a player's army are 'claimed' by the owning player, whilst all other buildings are 'unclaimed'.
• A claimed building is a unit in the controlling player's army and will remain so, even if it later becomes unoccupied, until the building is either destroyed or claimed by an enemy.
• If a unit enters a building, they immediately capture and claim that building, and it becomes part of that unit's side until the building is either destroyed, or an enemy unit reenters it (and therefore re-claims it).