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.