[Permissibility of building bath facilities from a mosque’s rubble]

Answer (and Allah grants success): There is no objection to that; for no sanctity remains in the rubble once detached. Sanctity pertains to the place. As for walls, roofs, and doors, their sanctity is only due to their connection to and inclusion within the place; when they are removed, their sanctity ceases, and selling them upon being surplus is valid. Were they to retain the mosque’s ruling, such a sale would not be valid. The rubble has no sanctity in itself and is not described as a “mosque” when separate; hence, the evidences for the inviolability and honor of mosques do not apply to it.
Accordingly, there is no objection to building bathrooms and facilities with the surplus rubble. Allah knows best; praise belongs to Allah, Lord of the worlds.
Source : Min Thimār al-ʿIlm wa al-Ḥikmah vol.1