His father built a house with a riba-based loan. Should he get married in it?

If your father took this loan before he found out that it is haraam to take a riba-based loan, then we ask Allaah to pardon him, but he has to repent to Allaah for failing to seek essential knowledge and ask those who had knowledge. There is no sin on you for living in this house and making use of the room, but you should resolve not to engage in riba again, and to ask the people of knowledge before entering into any financial transactions that are common nowadays and which may involve kinds of haraam things of which many people are unaware. 

The Standing Committee for Issuing Fatwas was asked about a person who took out a riba-based loan and built a house with it, and he is asking whether he should demolish the house, although he took the loan not knowing that it was haraam. 

They replied: If the situation is as described, what you got from borrowing money in this manner is haraam, because it is riba, and you have to repent and pray for forgiveness for that, and regret what you did, and resolve not to do anything like that again. As for the house which you built, do not demolish it, rather benefit from it by living in it or otherwise, and we hope that Allaah will pardon you for your mistake. End quote from Fataawa al-Lajnah al-Daa’imah (13/411) 

We ask Allaah to help you to get married and to make easy for you the means of goodness and success. 

And Allaah knows best.