Am I obligated to pay the cost of changing the staircase steps in the sealed apartment, or am I not required to do so?

The ruling on improving and modifying common parts of a property varies depending on custom and the laws regulating construction. If there is no custom or law, the default is that whoever desires the alteration bears the cost alone, and his partner cannot be compelled to participate, because the Prophet, peace and blessings be upon him, said: "The property of a Muslim person is not lawful except with his willing consent."