Party wall: merely placing a building against it does not oblige the purchase of joint ownership

Imagine: you have just bought a house with a garden in Salon-de-Provence. You decide to build a small extension along the wall separating you from the neighbour. Nothing exceptional, you think. But then your neighbour demands payment of half the wall, claiming that your construction “encroaches” on his property. Must one necessarily pay to use a party wall? Hundreds of owners ask themselves this question every year. The judgment of the Court of Cassation of 18 February 1987 (no. 85-16.657) provides a clear answer: the mere juxtaposition of a building against a party wall is not sufficient to constitute an encroachment justifying the obligation to acquire joint ownership. In other words, you are not obliged to buy half the wall simply because you build against it.

The facts: a story like many that happen every day

Mrs X. owns a house in Salon-de-Provence, with a garden enclosed by a party wall with her neighbour, Mr Y. This wall, several decades old, separates the two properties. Mrs X. decides to carry out thermal insulation work: she has a trench a few centimetres deep dug along the wall, on her side, to insert polystyrene panels. Nothing more. But the neighbour considers himself wronged: in his view, by digging and applying the insulation, Mrs X. has “encroached” on the common wall, which would oblige her to acquire joint ownership (i.e., to pay half the value of the wall). The dispute arises.

Mr Y. sues Mrs X. before the tribunal de grande instance of Aix-en-Provence. He claims payment of half the wall, valued at several thousand euros. The court rules in his favour: it orders Mrs X. to acquire joint ownership of the wall. Mrs X. appeals. The Court of Appeal of Aix-en-Provence upholds the judgment. Determined not to give in, Mrs X. appeals to the Court of Cassation. She argues that the mere juxtaposition of a building (or here, insulation) against a party wall does not constitute an encroachment justifying the obligation to acquire joint ownership.

The Court of Cassation rules in her favour. In its judgment of 18 February 1987, it quashes the appeal judgment and refers the case to another court. The message is clear: one cannot force an owner to buy back a wall simply because they have touched or minimally used it.

The court's reasoning — dissected

To understand, we must return to the law of party walls. A wall is said to be a party wall when it belongs half to each of two neighbouring owners. In principle, each owner may use the party wall, but only on condition of not “encroaching” on the neighbour's part. What is an encroachment? It is a use that goes beyond mere tolerance: for example, digging a cavity in the wall, installing a beam in it, or permanently supporting a construction on it. But the law (Article 661 of the Civil Code) provides that if an owner makes an “encroachment”, the neighbour may oblige him to acquire joint ownership of the wall. Note however: not every contact constitutes an encroachment.

In this case, the appeal judges had considered that the mere fact of digging a trench a few centimetres deep and inserting polystyrene panels constituted an “encroachment” on the party wall. The Court of Cassation contradicts them. It reminds that the juxtaposition of a building (or insulation) against a party wall is not sufficient in itself to constitute an encroachment of a nature to justify the obligation to acquire joint ownership. What few people know is that the Court relies on a strict interpretation of encroachment: it requires an appropriation or physical modification of the wall, not mere contact.

In clear terms, the decision confirms consistent case law: the neighbour's right of property is not violated by a simple support or added insulation against the wall. But what exactly does this change? This means that owners can carry out insulation work, install a lightweight fence, or even build a small extension against a party wall, without fear of being forced to buy it.

What this means for you — concretely

For the landlord: you own a rental property in La Ciotat, with a party wall. You want to install external insulation cladding. Good news: you do not have to pay for joint ownership of the wall, unless you dig into the wall or embed elements in it. Example with figures: a party wall of 20 linear metres may be worth €5,000 (€250/m). If the neighbour demanded acquisition, it would cost you €2,500. Thanks to this decision, you can save that amount.

For the buyer: before buying a house, check if there are any constructions against the party wall. If the seller has already carried out works without acquiring joint ownership, you could inherit a dispute. If you are in this situation, ask for a warranty clause in the deed of sale.

For the tenant: you want to put a shelf or piece of furniture against a party wall? No problem. But if you drill the wall to fix a curtain rod, beware: this could be considered an encroachment. Better to ask the owner's permission.

For the co-owner: in a condominium, party walls between units are often governed by the regulations. This case law does not apply directly, but it can influence court decisions in disputes between co-owners.

Four tips to avoid this type of dispute

• Consult the title deed and cadastral survey before any work against a wall. Check whether the wall is a party wall, privately owned, or common. A simple cadastral extract can save you years of litigation.
• Draft a party wall agreement if you plan significant works (e.g., building an extension). Agree with the neighbour on the terms of use and maintenance of the wall. Example: in Salon-de-Provence, two neighbours signed an agreement providing for sharing the costs of repairing the wall 50/50.
• Use a surveyor to precisely demarcate the properties. Often, disputes arise from confusion over the location of the boundary.
• Photograph and notify your works by registered letter to the neighbour. In case of a challenge, you will have evidence of the nature of the works (mere juxtaposition, no encroachment).

In-depth: related case law and developments

This 1987 decision is part of a consistent line of authority. For example, a judgment of the Court of Cassation of 12 May 1975 (no. 73-14.562) had already held that supporting a construction against a party wall does not create an encroachment. More recently, the Court of Cassation clarified (Civ. 3e, 10 June 2015, no. 14-17.193) that encroachment requires “incorporation” into the wall, such as embedding. The trend is therefore clear: the courts protect the freedom to use one's own land, provided the party wall is not physically altered. What does this development mean? If you face an abusive demand from your neighbour, you can rely on this case law to resist. undefined, I have encountered cases where a neighbour claimed several thousand euros for a simple coating applied to the wall. The 1987 judgment enabled us to defeat that claim.

What you absolutely must remember

FAQ:

• Can I build a wooden terrace against a party wall? Yes, if the terrace is simply placed on the ground and does not structurally rely on the wall. If you embed beams in the wall, that is an encroachment.
• What if my neighbour demands payment for joint ownership after my insulation works? Reply in writing, citing the 1987 judgment. Propose mediation. If refused, consult a solicitor.
• What are the risks if I do not pay? The neighbour may sue you. But if it is a mere juxtaposition, you have a strong chance of winning.
• Must I necessarily have a written agreement before gluing insulation against a party wall? No, but it is recommended to avoid any misunderstanding.
• Can my neighbour forbid me from planting vegetation against the party wall? No, planting does not constitute an encroachment, unless it penetrates the wall (e.g., ivy).

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