The first thing to do is to read the deeds and the co-ownership rules. Where they are silent or contradictory, the law lays down precise presumptions: the ground, the structural work, certain common equipment, the ducts, the passageways and several elements of a balcony or terrace are presumed common. A pipe connected to a piece of common equipment may remain common even where it runs through a flat.
What is a private part?
Co-owners may freely use their private parts and the common parts, provided they infringe neither the rights of the other co-owners nor the purpose of the building. (loi du 16 mai 1975, art. 2)
So the freedom exists, but it exists inside a shared building.
Which items are presumed common?
Where the deeds do not say otherwise, the law presumes common in particular:
- the ground, the courtyards, the parks and gardens and the access ways;
- the structural work;
- the items of common equipment, including the sections of pipework connected to them where these run through private premises;
- the ducts, casings and chimney heads;
- the premises housing common services;
- the passageways and corridors;
- the terraces, balconies, balustrades and railings, except the surface covering of terraces and balconies that is not visible from the outside.
(loi du 16 mai 1975, art. 3.2)
That list explains why "it is inside my flat" is not always a sufficient answer.
And a wall between two flats?
Where a partition or dividing wall does not form part of the structural work and separates two private parts, the law presumes it to be jointly owned between the premises it separates. (loi du 16 mai 1975, art. 3.4)
That matters before making an opening or carrying out alterations.
How is the share calculated?
Where the deeds are silent, the share is determined by the relative value of the private parts, assessed by reference to their make-up, their surface area and their location when the co-ownership was created, without regard to how they are used. (loi du 16 mai 1975, art. 6)
Ownership of an item and the way an expense is allocated nevertheless remain two different questions.
Can the co-ownership rules provide for anything at all?
No.
They bind owners and also tenants and occupants, but they cannot restrict co-owners' rights beyond what is justified by the purpose of the building. Certain clauses foreign to their subject matter are deemed unwritten. (loi du 16 mai 1975, art. 10)
The rules must therefore be read, but not treated as a text capable of inventing any rule at all.
Does a long-standing habit automatically change ownership?
No, not automatically.
Long-standing use of a space does not replace reading the deeds. Whether prolonged use may, in a particular case, produce an acquisitive effect is a matter for legal analysis and, where applicable, for case law. Use, even long-standing and undisturbed, is therefore not in itself proof of ownership.