Can a neighbor convert hallway to private space?

Unanimity is required. Converting a common area into a private area for private use is not an ordinary decision: it requires the agreement of all the co-owners (art. 22), not just a majority. The law classifies this operation among the most significant decisions, on the same level as constructing a new building, adding a story, excavating, or completely demolishing the building.

There is an important limit: this conversion is prohibited when it concerns the common areas listed in article 4 (for example the major structural works, the foundations, the load-bearing walls, the façade, the stairs, entrances, and corridors intended for common use, the common elevators). A hallway intended for common use falls precisely within these areas, which makes the operation very difficult, if not impossible.

Also bear in mind: the common areas and their accessory rights cannot be subject to division, seizure, or transfer independently of the private areas (art. 7). In other words, no one may appropriate a common area for themselves alone. If your neighbor acts without this unanimity, you may bring the matter before the court of first instance of the place where the co-ownership is located, which has jurisdiction over any dispute relating to this law (art. 59 undecies).