Short answer. Yes, but you must pay in. Under the Civil Code, an owner who did not contribute to increasing a party wall's height, depth, or thickness may still acquire part-ownership of the added portion — by paying his proportionate share of the cost of that work and of any land used for the increased 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 →
You can buy into the raised part
A party wall is owned in common by the adjoining owners, but when one of them raises it alone, he initially bears both the cost and the ownership of the added portion. Article 665 keeps the door open for you. It says the other owners who did not contribute to giving increased height, depth or thickness to the wall may, nevertheless, acquire the right of part-ownership therein. So your neighbour's decision to build higher at his own expense does not permanently lock you out of the added height. The law gives you a route to share in it.
The price of admission
That route is not free. To acquire part-ownership of the raised portion you must pay proportionally the value of the work at the time of the acquisition and of the land used for its increased thickness. In plain terms, you reimburse your fair share of what the improvement cost, measured as of when you buy in, plus your share of the value of any additional land the thickening required. The neighbour who fronted the expense is thus made whole for the part you take over. You gain rights in the added wall by contributing to what it took to build it.
Why the law lets you in later
The rule reflects the shared nature of a party wall. Because the wall stands between two properties and serves both, the law prefers letting a co-owner participate in improvements rather than leaving a permanent split between an owned lower half and a one-owner upper half. Allowing you to buy in — on fair terms to the builder — restores the common ownership across the whole wall. It balances the interests: the neighbour is compensated for his outlay, and you are not shut out of a structure that borders your own land.
What this does not let you do
The article gives a right to acquire part-ownership by paying; it does not let you simply use the added height for free, nor build on or load the raised portion before you have paid your share. Until you exercise the option and reimburse the proportionate cost, the added part remains the contributing neighbour's. The provision also assumes a genuine party wall held in common, not a wall standing entirely on one owner's land. And it settles the cost of buying in, not separate disputes over the wall's stability, boundaries, or maintenance.