A property-holding company (SCI) owns the only private unit in one building of a co-owned complex. It wants to add storeys to that building to create flats; the general meeting refuses; it goes ahead and starts the works. The Cour de cassation (the French supreme court for civil matters), in a reported decision of 2 April 2026, says who owns the right to build upwards when the co-ownership rules (règlement de copropriété) do not say: the co-owners’ association (syndicat des copropriétaires). That right can be sold, and voted on, by a qualified majority; it therefore has a price, and that is where valuation comes in.
The facts
In a complex of four buildings under the French co-ownership statute, an SCI owned unit no. 171, in building D, together with a share of the general common parts and all the special common parts of that building. It wished to add storeys to building D to create new private units and put the project on the agenda of a general meeting. The resolutions were rejected; it sought their annulment in court, while the association counterclaimed for the premises to be restored to their former state, the SCI having begun the works.
On 20 March 2024 the Paris Court of Appeal dismissed the SCI’s claims and ordered, under a periodic penalty, the cessation of the works and the restoration of the roof. The SCI appealed: unless the rules provide otherwise, the right to add storeys to a building used privately by a single co-owner would belong to that co-owner alone.
The decision
The Cour de cassation dismissed the appeal (Cass. 3e civ., 2 April 2026, appeal no. 24-15.059, reported).
It held that it follows from articles 3 and 35 of the Law of 10 July 1965 that, “where the co-ownership rules are silent, the right to add storeys, in order to create new private premises, to a building which includes common parts, even special ones, belongs to the co-owners’ association” (translated from the French). The Court of Appeal had found that building D consisted of the SCI’s unit and common parts, “from which it followed that the building was not a private part”, and held that the right to build upwards was not included in the special common parts of that building as defined by the rules. It “rightly concluded that only the co-owners’ association could sell the right to build upwards on building D, which belonged to it, and could authorise such works at a general meeting”.
What this changes for the valuation
A right that is sold. Article 35 of the 1965 Law allows the association to sell the right to build upwards, by decision of the general meeting at the majority of article 26; the proceeds go to the association and are shared among the co-owners. Since the Law of 24 March 2014, top-floor co-owners have a priority right to acquire it. Before voting, the meeting needs a price; after the vote, the top-floor co-owner needs to know whether to exercise the priority at that price. In both cases, the valuation of the right to build upwards is the starting point.
The method: the land charge. The right to build upwards compares with nothing that is commonly sold; it is measured by what it allows to be built. The expert starts from the floor area that can be built on top, according to the local planning plan, the permitted height and the structure of the building, then calculates the value of the units created on completion, from sale prices in the area for new or renovated flats on upper floors. From that are deducted the cost of the rooftop works, higher than ground construction because of structural reinforcement, the roof and access, fees, financing costs, contingencies and the developer’s margin. The balance is the land charge (charge foncière), what a developer can pay for the right to build. The Charte de l’expertise en évaluation immobilière (the French property valuation charter) describes this method, known as the developer’s residual appraisal (6th edition, November 2025, Title III, ch. 2, § 2.8).
What the association adds to the account. A rooftop extension changes the co-ownership shares, creates new charges, sometimes requires reinforcement of common equipment, and may reduce the value of the existing top-floor units, which lose view or sunlight. These elements enter the negotiation of the price, and a report that identifies them prevents them from resurfacing after the vote.
The case of a building owned by a single co-owner. The decision concerns every complex where a building belongs to one co-owner but includes common parts, even special ones: the right to build upwards is not theirs, unless the rules say so. Such a co-owner who wants to build upwards will have to buy the right from the association; the price is discussed on the basis of the land charge, not the cost of the roof.
What the expert takes from it
- Absent a contrary clause in the rules, the right to add storeys to a building with common parts belongs to the association, even where a single co-owner holds the units in it.
- The right is sold by a vote at the majority of article 26; the top-floor co-owner has a priority to acquire it.
- It is valued through the land charge: value of the units created, less construction costs, fees and margin.
- The report takes in the consequences for the existing units and the common equipment.
- A rooftop extension begun without the vote leads to restoration under penalty; the valuation comes before the works, not after.
Further reading
The Market value page describes the assignment, its timescale and its fee. The glossary entries charge foncière and bilan promoteur complement this article. On the same theme: Valuing the common parts of a building and Common areas sold for one euro by the managing agent: the fair price and the expert. The decision is available on Légifrance.
What next
A rooftop extension in prospect, or a right to build upwards to sell?
I value the right to build upwards through the land charge: what the new units will be worth, less what they will cost to build, with references from the area, so that the general meeting votes on a figure.
Free quote, by email or by phone. No commitment before the quote is accepted. Fees are never linked to the value of the property (Charte de l’expertise, Title I, §2.1).
