We live in a multi-family apartment in Israel (just beyond the green line). Our apartment building was built on a hillside so it is set up in such a way that all minus floors (we live on fl. -2) have a Chatzer (large yard). We discovered that located directly under our house was a large empty space that we could use for a second story to our apartment if we dug in our yard. Now that we have completed digging beneath our apartment I was warned that we should check whether or not this space really belongs to us or if we would have to share some of the space with the other 20+ neighbors or possibly reimburse them somehow for its use. A few points to consider: 1. The challal is located directly under our yard\apartment with no other neighbors having access to it. 2. We live in a city in which it is very common for people to use challal for their own apartments in a similar way. 3. The city does not offer building permits for using this space so it is widely known that people build without these permits. I would appreciate any insight that you may have on this including halachic sources or opinions of various Israeli Rabbi’s on this topic if known.

The question is complex and it is not advisable to give an answer to a specific case without knowing the reality in terms of the law, the purchase contract, the boundaries of the plot, the neighbors' claims, and more. It touches on the laws of neighbors, theft, between man and his fellow - relations with neighbors, Dina Demalchuta and community regulations. I would like to point out a number of approaches among Halachic authorities regarding the definition of spaces in a shared building in order to broaden the knowledge on the subject. But for practical ruling and also as good advice to avoid neighborly disputes, consult with the local rabbinical court. Summary of approaches in defining spaces in a shared building: 1) One approach is to view them as ownerless property, because they were not included in the sale, not by the seller nor by the purchasers of the apartments, and whoever is first to acquire it is entitled to it (משפט צדק בשם י"א). 2) A second approach entitles the residents who live beside the spaces from the time of purchase, by virtue of a compelling assumption that the space was sold to them, because it is of great benefit to them, and because they are the only ones who will be able to obtain a permit from the authorities to build there. (משפט צדק בשם הגר"נ קרליץ ובשם הגר"מ שפרן). 3) A third approach sees them as shared property, but opines that since the other residents cannot exercise their right to build the spaces, they are subjected to the law of "Kofin Al Midat Sedom" (we block attempts to behave as in Sedom) for the benefit of the residents who reside beside the spaces. (משפט צדק בשם הגרז"נ גולדברג). 4) A fourth approach considers them to be shared property for all intents and purposes and one cannot forcibly expropriate the ownership of the other residents; rather one should obtain explicit permission from them, if necessary giving them compensation. (שמרו משפט בשם המקורות הבאים: ספר עמק המשפט ח"ג, שכנים סימן נח. עטרת חכמים להרב ברוך טעם, השמטות סימן כא, והובאו דבריו בספר אבן שהם סימן קעא. ושכן מורים בהרבה בתי דין). (להרחבה ראה ספר משפט צדק בעניני הבית המשותף בהלכה להרב אילן אש. וספר שמרו משפט להרב שלמה זעפרני שליט"א ח"ב, בענין שותפים בקרקע, סימן לז, קביעה הלכתית בעניין חללים בבית משותף).