Is it permissible for an engineer to design buildings that contain places designated for discos and alcohol consumption? And what is the ruling concerning buildings that have already been designed?

Designing halls and places specifically for prohibited dancing, drinking alcohol, and the like is not permissible in Islamic law, because it constitutes clear assistance in sin, and Allah has forbidden that.

Whoever does this must immediately repent and resolve not to return to it.

Money received for designing a bar or dance hall is considered unlawful money, and one must dispose of it by spending it on general Muslim welfare.

If the amount is for designing the entire building, and it includes a hall for alcohol and dancing, then the value of this hall should be estimated, and that portion must be disposed of by spending it on general Muslim welfare, such as the poor and needy. The remaining amount is permissible for him.