What is the ruling on an architect's work designing and supervising the construction of residential compounds that contain swimming pools, knowing that he does not have the decision to add them and does not know who will use them in the future? And is he sinful if these swimming pools are used in a manner that does not observe proper concealment (ستر)?

The basic principle is that building residential complexes and swimming pools is permissible. However, selling, building, or assisting in their construction becomes forbidden if it is known that they will be used for revealing 'awrat (private parts) or for unlawful mixing (ikhtilat). This is because the means take the ruling of their ends, and whatever leads to the unlawful is likewise unlawful. It is also impermissible to sell items that can be used for either lawful or unlawful purposes if it is highly probable that the buyer will use them for unlawful purposes. Given that the situation of those who will reside in these apartments and use these swimming pools is unknown, there is no harm in building them or assisting in their construction, unless there is certainty or a strong presumption that they will be used for unlawful purposes.