The question of who has the right to add a storey to a co-owned building continues to occupy the courts. The third civil chamber of the Court of Cassation, in a decision published in the Bulletin (Cass. 3e civ., 2 April 2026, n° 24-15.059), provides a clear answer: this right belongs, unless otherwise stipulated, to the co-ownership syndicate, even when the building concerned comprises only a single private unit.
In this case, a civil real estate company, owner of the sole unit in a building that was part of a co-owned complex, had undertaken steps to proceed with works to add an extra storey. Believing that the building belonged to it entirely, it claimed the freedom to exercise this right alone. The co-owners' association, contesting this analysis, brought the case before the judge. The Paris Court of Appeal having granted the co-owners' association's request, the SCI lodged an appeal, arguing that ownership of the sole unit should entail ownership of the volume situated above.
The Court of Cassation dismisses the appeal. It recalls, under Article 35 of Law No. 65-557 of 10 July 1965, that the construction of additional storeys on a building comprising common areas, with a view to creating new private premises, may only be carried out by the co-owners' association, by a majority vote as stipulated in Article 26. However, even if the SCI owned the sole unit in the building, this unit included special common areas – notably the structural work, foundations, and roof. The building could therefore not be deemed to be a purely private part, and the airspace above it could not be considered independently of the collective regime. The decision to build additional storeys, and if applicable the sale of this right to a third party, therefore falls within the exclusive competence of the general meeting.
This ruling usefully reminds us that ownership of a unit, even if it's the only one, does not grant the right to exploit the airspace above the building. A co-owner wishing to build an extra storey must obtain express authorisation from the general meeting, under the conditions laid down by the law of 10 July 1965 and its implementing decree. Otherwise, work undertaken without authority is liable to demolition proceedings initiated by the syndicate, not to mention the risk of compensation and the practical impossibility of obtaining a building permit cleared of any appeals. The decision also invites the authors of co-ownership rules – particularly in complex property developments – to anticipate the issue, by means of an express clause reserving or assigning the right to build an extra storey.
Before any extension operation, it is therefore essential to carefully examine the co-ownership rules, to have the exact composition of the building diagnosed (general or special common areas, private areas), and to prepare in advance the resolution to be submitted to the assembly. Otherwise, the project, however well-developed technically and financially, will remain legally compromised. The firm is available to co-owners, syndicates, and investors wishing to secure the legal feasibility of an extension operation.