Is it permissible to build a health unit in one of the corners of a building for memorizing the Holy Quran, located on the roof of the village mosque, given that its drainage is external and all necessary precautions have been taken to prevent any leakage?

It is not permissible to build a healthcare unit above a mosque because this was not the intention of the endower (waqif), unless the endower had intended it at the time of endowment, in which case some scholars permit it. It is recommended to restrict building above the mosque to a center for memorizing the Holy Quran, which would then take the ruling of the mosque, as everything above a mosque takes its ruling. However, if the healthcare unit means restrooms, then the prohibition is more emphatic because it was not the intention of the endower, and because it would place the mosques beneath restrooms.