Is it permissible to move the apartment door and incorporate the small area in front of it into the apartment, knowing that no one other than us has used it for thirty years, and it will not harm any of the building's residents?

If the apartment is rented, it is not permissible to annex a portion of the staircase to it except with the landlord's permission. If it is owned, it is not permissible to annex this area except with the permission of all owners, because the staircase and its facilities are common property for everyone. Therefore, it is not permissible to take any part of it except with their consent. This is similar to a road or a cul-de-sac, which is the property of all the residents of that road. It is not permissible to make any alterations to it except with their permission, and this includes not building a balcony (rawshan), a covered passageway (sabāt), a bench (dakka), or a a drain (mīzāb). And if it is stipulated to the buyer that no alteration should be made to the building's appearance, then it is not permissible to make any alterations even if the neighbors agree, based on the words of Allah Almighty: (O you who have believed, fulfill [all] contracts) and the words of the Prophet, peace and blessings be upon him: (Muslims are bound by their conditions).