Is it permissible to secretly buy land and build on it without the knowledge of the father or the wife's brothers?

There is no blame upon you for preparing for the future or for concealing the matter from your father, because a wife has the right to an independent dwelling, and she is not obliged to live with her husband's family. Obedience to parents is obligatory within what is known to be good, beneficial to them, and does not cause harm or hardship to the child.