Assalaamu alaykum. I may have a job opportunity to work for a company that provides engineering consulting services where I would design systems such as heating, air conditioning, plumbing, and fire safety systems for new and existing buildings. I do not know what projects I will be assigned, but some of these projects may include things such as hotels, restaurants, concert halls, and theaters. These places, when finished, may be used for haram activities such as alcohol consumption, but would it be haram for me to design mechanical systems for the buildings themselves? I am not directly taking part in any haram activities that may happen in the building once it is finished, but is designing the building considered cooperating in their sin and would it be haram? May Allaah reward you.

All perfect praise be to Allaah, The Lord of the worlds. I testify that there is none worthy of worship except Allaah and that Muhammad, sallallaahu ʻalayhi wa sallam, is His slave and Messenger. 

It is absolutely prohibited to take part in the construction of buildings that are particularly designated for prohibited activities, such as gambling casinos, bars, and the like, because this would constitute cooperation in committing sins, and helping others to disobey Allaah is prohibited. Allaah, The Exalted, says (what means): {And cooperate in righteousness and piety, but do not cooperate in sin and aggression. And fear Allaah; indeed, Allaah is severe in penalty.} [Quran 5:2]

For more benefit, please refer to fataawa 82832 and 92732.

As for the buildings of which it is doubted whether they will be designated for prohibited activities or not, it is not forbidden to take part in their construction. This is not considered prohibited cooperation in committing sin because the basic principle is that all things are lawful unless proven otherwise, and mere doubts do not entail prohibition.

It is also allowable to take part in constructing buildings that are not designated for prohibited activities even if some forbidden actions are going to be carried out in them, such as shops that are designated for selling lawful commodities but may also sell some prohibited items as well.

Allaah knows best.