Is it permissible to pray in a place originally designed to be a lavatory, but whose design was then changed to become a room before the house was built?

There is no objection to praying in the place designated for a future restroom if it is clean, because the Prophet, peace and blessings be upon him, said: "The entire earth is a mosque, except for the graveyard and the hammam (bathhouse)," narrated by Ahmad and Tirmidhi with a sound chain of transmission. A place is not considered a place for relieving oneself unless it is prepared for that purpose, not merely by the suggestion that it be a restroom.