You can only cast this spell within a constructed room expansion of a structure you own, which cannot be the bonus expansion of the structure. Attempting to cast it elsewhere will cause the spell to fail, its mana expended.

Choose another room expansion you would otherwise be able to construct, with the same number of room points as the room expansion you cast this spell in. After the spell is completed, the room expansion you cast this spell in is transmuted into the room you chose, this new room expansion’s benefits replacing those of the old roomIf the new room expansion would have a greater construction cost than the old room, the materials cost of this option includes an amount of precious metals (such as gold, mithral, or adamantium) equal to the difference in construction costs, which the spell consumes. If these extra material components are not present, the spell fails, its mana expended.