Short answer. Under Article 666, every part-owner of a party wall may use it in proportion to the right he has in the co-ownership, but only without interfering with the common and respective uses of the other co-owners. Your share of use tracks your share in the wall, and it stops where it disturbs the others.
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 →
Use in proportion to your share
A party wall is a wall shared between two properties and owned in common by the adjoining owners. Article 666 fixes how much each may use it. It provides that 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. The first measure is your proportionate right. If ownership of the wall is shared equally, your use rights are correspondingly equal; if the shares are unequal, so are the entitlements. Use is not open-ended — it is measured against your stake in the co-owned wall.
The non-interference limit
The proportion is only half of the rule. The second, equally important condition is that your use must be without interfering with the common and respective uses by the other co-owners. So even within your proportionate share, you may not exercise your use in a way that blocks or damages the others' own use of the same wall. The two limbs work together: you are entitled to use the wall according to your share, and you must exercise that use without prejudicing your neighbours. A use that is technically within your proportion but effectively deprives a co-owner of his use crosses the line the article draws.
What using a party wall can include
In everyday terms, using a party wall may mean resting beams on it, building against it, or otherwise relying on it as support for your structure, consistent with your share and the others' rights. Because the wall serves both properties, the law expects a give-and-take: each owner draws on the wall while leaving room for the other to do the same. The article states the governing principle rather than a detailed catalogue of permitted acts, so what counts as proportionate and non-interfering depends on the situation of the particular wall and the buildings it supports. Reasonableness toward the co-owner is the guiding idea.
Where disputes arise
Friction usually comes not from the proportion itself but from the interference limit — one owner loading, altering or extending the wall in a way the other says harms his use. The article does not, by itself, spell out the procedure for repairs, alterations or the costs of maintaining a party wall, which other provisions on party walls and co-ownership address. What Article 666 settles is the basic entitlement: each part-owner may use the shared wall in line with his share, provided he does not trample the equal rights of those with whom he shares it. That balance is the heart of party-wall use.