demolish the new structure he has erected?

He answered:

If he is in possession of the upper storey and the owner of the lower storey does not claim it as his own, it belongs to the person in possession until someone else establishes proof that it belongs to him. As for the new construction he has added, he has no right to do so unless this falls within the rights attached to his property. And Allah knows best.

He was also asked:

A man purchased an upper storey that did not previously project outward. He then renovated it and added a structure projecting over a cul-de-sac shared with his neighbours. He claimed that the alley had a doorway east of al-Zāhiriyya. Is he entitled to add this projecting structure?

He answered:

Praise be to Allah, Lord of all worlds. He is not entitled to add a projecting structure over a passage that has no outlet, by agreement of the imams; they have not disputed this. They have, however, differed over whether such a structure may be added over a through passage. That issue involves disagreement and distinctions that are beyond the scope of this discussion.

As for a passage that has no outlet, there is no disagreement concerning it, whether or not it has a doorway leading to a school, for it is not a through passage. If he claims that he has the right to maintain a projecting structure there, his claim is not accepted without proof. He may, however, require the neighbours who dispute his claim to swear an oath denying his entitlement to it. And Allah knows best.