Short answer. Only a proportionate part. Article 666 of the Civil Code limits each part-owner's use of a party wall to their share in the co-ownership. No co-owner may use the wall in a way that interferes with the common and respective uses of the other co-owners.
What the law says
Every part-owner of a party wall may use it in proportion to the right he may have in the co-ownership, without interfering with the common and respective uses by the other co-owners.
Civil Code, Article 666 — Use Of A Party Wall. Read the full provision →
What a party wall is
A party wall is a wall that stands on the boundary between two adjoining properties and is co-owned by the owners of those properties. Each owner holds a proportionate share in the wall — usually half — with the wall jointly supporting or separating their respective structures. Because neither owner has the wall entirely to themselves, the Civil Code governs how each may use it so that neither's exercise of ownership prejudices the other.
Use is proportional to ownership share
Article 666 ties each co-owner's right to use the wall to the right they hold in the co-ownership. If you own half the party wall, you may use it to the extent of half — you cannot build into the full thickness as if the wall were entirely yours. Using more than your share to anchor beams, embed pipes, or support an extension would infringe on the other co-owner's portion and is not permitted without their consent.
The non-interference requirement
Even within the limits of one's proportionate share, Article 666 requires that the use must not interfere with the common and respective uses by the other co-owners. The wall serves both properties. If your planned use — say, embedding a heavy steel beam at the full depth of the wall — would undermine the wall's structural integrity or compromise how the other co-owner uses their side, that use is prohibited. The limitation applies not just to the physical material of the wall but also to how it functions for everyone.
Getting consent for expanded use
If you need to use more than your proportionate share — for example, to anchor a load-bearing beam that requires penetrating the other side — you must obtain the other co-owner's consent. If they agree, document that agreement in writing. If they refuse and your proposed use is necessary and reasonable, there may be legal remedies, but unilaterally proceeding over objection exposes you to liability for any damage caused and to a claim for restoration of the wall to its prior state. Resolve these matters with your neighbour before construction begins.