Short answer. Yes. Article 665 of the Civil Code allows the other co-owner — who did not contribute to raising the wall — to acquire part-ownership of the raised portion at any time by paying the proportional value of the work at the time of acquisition plus the value of any additional land used for greater thickness.
What the law says
The other owners who have not contributed in giving increased height, depth or thickness to the wall may, nevertheless, acquire the right of part-ownership therein, by paying proportionally the value of the work at the time of the acquisition and of the land used for its increased thickness.
Civil Code, Article 665 — Buying Into A Raised Wall. Read the full provision →
The right to buy in later
Article 665 of the Civil Code provides: "The other owners who have not contributed in giving increased height, depth or thickness to the wall may, nevertheless, acquire the right of part-ownership therein, by paying proportionally the value of the work at the time of the acquisition and of the land used for its increased thickness." The word nevertheless is the key: even though you did not share in the cost when the work was done, you are not permanently excluded. You may enter later — paying based on what the work is worth at the moment you buy in, not at the moment it was built.
What price is paid and how it is measured
The statute sets two components for the price. First, the proportional value of the work itself — the cost of raising the wall in height, increasing its depth, or expanding its thickness — calculated at the time you acquire, not the time it was built. If construction costs have risen in the years since your neighbor did the work, you pay the higher current value. Second, if the neighbor used land beyond the boundary line to increase the wall's thickness, you must also pay for the land corresponding to your proportional share of that increased thickness. Both components must be covered for the buy-in to take effect.
What you get and what remains separate
Exercising the right gives you part-ownership of the raised or thickened portion of the wall — the part your neighbor added at his own cost. This is separate from whatever co-ownership arrangement already existed in the original party wall. Once you buy in, you share in the portion proportionally: you share in its maintenance, in the benefits of using it to support your own structure, and in whatever obligations come with that ownership. Before you exercise the right, your neighbor's raised portion belongs to him alone, and he bears its upkeep.
Practical considerations
Because the price is measured at the time of acquisition, waiting to buy in is not necessarily advantageous — the longer you wait, the more the current market value of the work may diverge from what was actually spent. On the other hand, the right does not expire: the statute sets no time limit on when you may exercise it. If you wish to raise the wall yourself in the future to match your neighbor's height, you would need to factor in co-ownership of the raised portion, since exercising the buy-in right first may be the cleaner path.