Is it permissible for him to supervise the construction of a house for people who took out a riba-based loan from a bank?
Question

I am an engineer, and I have an engineering company in which I work. People come to me who have taken out riba-based loans from the bank, and these loans are for the purpose of building houses. They ask me to supervise the construction for which they took out these loans, or they consult me about construction matters, or any work connected to this construction, whether that is in return for payment or otherwise, and I do the work. Do I incur any sin by doing that, or am I encouraging people to deal with riba by doing this work? Please note that in the past I used to draw the plans on the basis of which they took out the loans, and I used to write reports at the end of each stage of construction, so that they could receive loan payments. When I heard a fatwa from one of my friends which stated that I was like the one who writes down the riba-based loan, I stopped doing that. Do I have to do anything else? If I have to do something, what is the expiation for that?
Praise be to Allah.We have no need to explain the ruling on their riba-based loans, as it is perfectly clear. Those who take out such loans have committed a major sin, and you should advise them and remind them that what they are doing is haraam and that they need to repent from it.
With regard to your question:
The work of a construction engineer may be haraam or it may be halaal:
If he designs the house for them or draws up plans for them so that they can obtain a riba-based loan, this action is haraam and he is helping them in sin and transgression, because his action in this case is connected to the riba-based loan. Allah, may He be exalted, says (interpretation of the meaning):
{And cooperate in righteousness and piety, but do not cooperate in sin and transgression} [al-Maa’idah 5:2].
But if the designing and drawing of plans came after they obtained the riba-based loan, then there is nothing wrong with the engineer designing the house, drawing the plans and supervising the construction, even if the funds of the owners were obtained through an unlawful riba-based loan, because the riba-based loan has to do with those people who dealt in riba, not the money itself, and the engineer is only accepting that money in return for his effort and his work. The same applies to the one who sold them the land or building materials; there is no blame on them for doing that, because they all received the money in return for what they sold of land and materials, and they have nothing to do with the riba-based loan.
See also the answer to question no. 82277 .
And Allah knows best.