If it is stipulated in the rental contract that you should not use any electrical appliances except those that are supplied with the accommodation, then you have to adhere to this condition, and it is not permissible for you to use your personal washing machine unless the people in charge agree to that, because the basic principle is that you are obliged to fulfil agreements and adhere to conditions. Allaah says (interpretation of the meaning): “O you who believe! Fulfil (your) obligations” [al-Maa’idah 5:1. And the Prophet (blessings and peace of Allaah be upon him) said: “The Muslims are bound by their obligations.” Narrated by Abu Dawood (3594). Classed as saheeh by al-Albaani in Saheeh Abi Dawood.
This condition that you mention is usually stipulated in student residences so as to avoid serious problems. Based on the large numbers of students in the residences, if each of them were allowed to use whatever electrical appliances he wants to, the circuits would not be able to bear this heavy load, which would lead to frequent outages and that would harm the students or cause a fire.
This confirms that you have to adhere to this condition.
And Allaah knows best.