What should one who watched pornographic films involving adultery in an office located within the perimeter of a mosque building, which will later become a mosque parking lot, do when they feel remorse and wish to return to their previous state?

Watching pornographic films corrupts the heart and distances one from Allah. It is forbidden, as Allah Almighty says: "Tell the believing men to reduce [some] of their vision." What you did does not affect the sanctity of the mosque. You must offer sincere repentance for watching these forbidden things by desisting, regretting, and resolving not to return to them. Fortify yourself with dhikr (remembrance of Allah) and du'a (supplication), and occupy your time with beneficial deeds.