Changing the use of private units in co-ownership: why unanimity is essential

One autumn morning in Paris, a co-owner discovered that his downstairs neighbour had converted his flat into an office without warning anyone. The smell of customers' coffee had replaced that of family croissants. Can he oppose this? What does the law say? This is precisely the question that the Court of Cassation settled in a landmark ruling of 2 October 1979. A ruling that, nearly fifty years later, remains of burning relevance for any owner of a co-ownership unit.

In this Parisian case, the stakes were high: a buyer wished to use his unit for residential purposes, whereas the co-ownership regulations (the contractual document setting out the rights and obligations of each co-owner) restricted it to professional activity. The lower courts had initially thought they could allow this, on the grounds that it only concerned private units. A fundamental error, retorted the high court: changing the use of a private unit strikes at the very essence of the building, and requires the agreement of all.

This principle, enshrined in the Law of 10 July 1965, protects collective harmony. But what exactly does it cover? And what are the practical consequences for an owner considering changing the use of his property, or for the co-owners' association that must enforce the regulations? Let us dissect this major decision and its practical implications.

Facts: a story that happens every day

The case began with the purchase of a flat in a Parisian building. The co-ownership regulations contained a special clause: the unit was intended for professional use, specifically as a "property manager". This wording appeared explicitly in the deed of purchase, and thus in the rights of the new owner. But the latter, wishing to make it his main residence, set about changing this use.

It appears that the co-owner had tried to obtain authorisation at a general meeting, without success. The matter was then brought before the courts. At first instance or on appeal, the court considered that the conversion was lawful: after all, as it concerned only private units, the majority will of the co-owners could suffice. A tempting line of reasoning, but incorrect in the eyes of the Court of Cassation.

The co-owners' association, representing the collective, lodged an appeal. The high court had to determine whether changing the use of a private unit, as defined by the co-ownership regulations, requires the unanimity of the co-owners. The answer would shape the contours of co-ownership life for decades to come.

The court's reasoning – dissected

The Court of Cassation quashed the appeal judgment on the basis of Article 9 of the Law of 10 July 1965 (the fundamental text governing co-ownership of built-up properties). This article provides that "the co-ownership regulations may be amended in their provisions relating to the use of the building or the rights of enjoyment of private units only with the unanimous agreement of the co-owners". A clear principle, admitting no exception when the amendment affects what is known as the "use" of private units.

To understand fully, one must distinguish two concepts: common areas (corridors, roof, entrance) and private units (flats, cellars, parking spaces). Whereas the former are by nature collective, the latter are reserved for the exclusive use of their owner. But the latter's freedom is not absolute: the co-ownership regulations may specify their use, i.e. the purpose for which the unit is intended: residential, professional practice, commercial… This use is a structural element of the co-ownership's equilibrium.

The judges censured the lower courts for having disregarded this principle. It matters not that the change concerns only private units: as soon as it affects their use as set out in the regulations, it requires unanimous consent. The court stressed that the disputed clause ("property manager") was a provision of the regulations relating to the rights of enjoyment of private units, and not a mere detail of the common areas. In short, the regulations were the law between the parties, and could not be departed from without the agreement of all.

This solution, although handed down in 1979, has never been called into question. It is a pillar of co-ownership case law. The Court of Cassation thus reminds us that co-ownership regulations are contractual in nature: they bind all the co-owners and form the framework of their respective rights. To touch them is to modify the contract, hence the requirement of unanimity. Moreover, this strictness is fully justified: it prevents circumstantial majorities from imposing on a reluctant co-owner a change of environment that could devalue his property or impair his quality of life.

What it means for you – in practice

This decision directly concerns you, whether you are an owner-occupier, a landlord, or a prospective buyer. Here are the practical repercussions.

If you wish to change the use of your unit – for example from residential to professional practice, or vice versa –, you must imperatively obtain the agreement of all the co-owners, without exception. A simple majority at a general meeting is not enough. Unanimity means that all co-owners, present or represented, must vote in favour. A single vote against blocks the project. In Paris, where buildings sometimes have dozens of units, this can be a difficult task.

Consequence for the buyer: before purchasing, read the co-ownership regulations meticulously. Check the designated use of the unit. A former commercial premises converted into a dwelling without authorisation may expose you to proceedings. Your notary should draw your attention to this point at the time of sale. A Parisian example: buying a duplex in the Marais is tempting, but if the regulations restrict it to professional use, you will not be able to live there without unanimous agreement.

For the co-owners' association, this decision is a weapon: you can demand that a unit whose use has been changed without unanimity be brought back into compliance. This applies to holiday lets such as Airbnb (if the regulations limit use to strictly residential) or to the installation of a business in a building intended exclusively for residential use. The courts generally order the offender to cease the activity under penalty (a sum per day of delay) and to pay damages.

Four tips to avoid this type of dispute

• Check the use clause in the co-ownership regulations before any project. This document, often bulky, holds the key to your rights. Have it reviewed by a professional if in doubt.
• Do not confuse a change of use with a simple allocation of common areas. If your project involves a change in the purpose of the unit (residential to professional), you fall within the scope of Article 9. Obtain total consensus.
• If unanimity is refused, explore alternative solutions. Can you carry out the activity without changing the use? A professional practice may sometimes be compatible with a residential clause if it does not cause nuisance and remains ancillary. But beware, this tolerance is fragile and depends on the wording of the regulations.
• Consult a specialist lawyer before any sensitive general meeting. Prior advice will save you months of proceedings and help you present a solid case to convince the co-owners.

Further insight: related case law and developments

The 1979 solution is part of a consistent line of case law. A few judgments deserve mention to gauge its scope. Thus, the Court of Cassation reaffirmed the principle in a judgment of 14 November 1991 (appeal no. 90-12.989): the conversion of a flat into commercial premises, in breach of the regulations, justifies action by the co-owners' association. More recently, a judgment of 24 September 2008 (appeal no. 07-15.590) reiterated that changing the use of private units, even minimally, requires unanimity, even if the new activity would be less bothersome than the old one.

The trend of the courts is towards strictness, thereby protecting the stability of the status of co-ownership buildings. This requirement of unanimity may seem excessive, particularly in small co-ownerships where a single dissenter blocks everything. But it has the advantage of protecting the rights of minorities. At a time when uses are evolving (teleworking, house-sharing, short-term rentals), the debate is ongoing. One thing is certain: without legislative reform, unanimity remains the golden rule.

Key takeaways

Here are the essential points to remember from this decision, in question-and-answer form.

What is the designated use of private units?
It is the purpose defined for each unit in the co-ownership regulations (residential, professional, commercial). It determines what you can legally do in your own home.

Can I change this use without authorisation?
No. Any change of use requires the unanimous agreement of all co-owners, in accordance with Article 9 of the Law of 10 July 1965.

What is the difference from common areas?
Common areas belong to everyone and their use is collective. Private units belong to you, but their use is governed by the regulations.

What do I risk for non-compliance?
The co-owners' association can take you to court to stop the prohibited activity, obtain an order to restore the premises under penalty, and claim damages. Legal costs are often high.

How do I obtain unanimity?
By convening an extraordinary general meeting with a proposal to amend the regulations. Unanimity means a favourable vote by all co-owners, including those absent or unrepresented.

Are you in a similar situation? A 30-minute initial consultation with Maître Zakine (£45) can save you months of proceedings – and often much more. Book an appointment →