Does working in the maintenance of a usurious bank entail a grave sin due to the unavailability of other work, and is it permissible to buy or build a house from the salary or savings in this situation?

Working in a usurious bank is not permissible except in cases of extreme necessity, such as not finding enough to sustain life or shelter. The sin is not limited to the usurer but extends to everyone who assists him. In Sahih Muslim: "The Messenger of Allah (ﷺ) cursed the one who consumes Riba, the one who pays it, and its two witnesses. And he said: 'They are all equal.'"