Praise be to Allaah.
What is obligatory on the wealth of the rich is zakaah; whatever is left after paying zakaah, even if it is a lot, is halaal (permissible) for them and it is permissible for them to buy a big house, but they should avoid extravagance because it is haraam in Islam even if the extravagant person is rich, because Allaah says (interpretation of the meaning):
“… and waste not by extravagance. Verily, He likes not those who waste by extravagance.” [al-An’aam 6:141]
It is mustahabb for them to give charity to the poor apart from zakaah, and the best charity is that which comes from richness. According to Islam, they are also obliged to honour their guests and help those who are in desperate need, and to give to poor people who ask for help – even if they have already paid their zakaah. And Allaah knows best.