Is it permissible to sell a residential apartment for commercial or administrative use, and does a single neighbor have the right to prevent that, claiming harm?

It is permissible to sell an apartment to an office, company, or clinic, provided that it does not cause significant harm to the neighbors, based on the Prophet's (peace be upon him) saying: "By Allah, he does not believe... whose neighbor is not safe from his evils," and Ibn Attab's statement that anything that harms a neighbor must be stopped.

If the custom in your country allows such activities in residential buildings, then there is no harm in selling. However, if people are harmed by them, then you are not permitted to sell for this purpose, in order to prevent harm to the neighbor.

Some scholars are of the opinion that a neighbor has the right of pre-emption (shufa'a) if neighbors share a common right, such as the building's entrance, based on the Prophet's (peace be upon him) saying: "The neighbor has more right to the pre-emption of his neighbor... if their path is one." He is then entitled to acquire the apartment at the price for which it was sold. You should offer the purchase to the neighbor before anyone else, as narrated by Muslim from Jabir (may Allah be pleased with him).

In summary: If the sale results in clear harm to the neighbors, it is forbidden. The neighbor has the right of pre-emption as long as there is a shared right between you. If the neighbor does not take the apartment and there is no significant harm, then there is no basis for their objection.