Short answer. Two or more people owning one undivided thing. That is the whole test — the thing is not split into parts, so each co-owner holds a share in the whole rather than a particular corner of it. No agreement is needed for co-ownership to exist; it arises from the fact itself.
What the law says
There is co-ownership whenever the ownership of an undivided thing or right belongs to different persons.
Civil Code, Article 484 — What Is Co-ownership. Read the full provision →
The definition turns on one word
Article 484 provides that there is co-ownership whenever the ownership of an undivided thing or right belongs to different persons. The word carrying the weight is undivided. Where a lot has been subdivided and each owner has his own parcel with its own title, there is no co-ownership at all — there are neighbours. Co-ownership is the state of affairs where the thing has not been split, so nobody can point to a portion and say that this part is his. What each holds is an abstract share in every square metre of it.
It arises by operation of fact, not by choice
Nobody has to sign anything. Heirs become co-owners of what the deceased left the moment he dies, before any settlement is drawn up. Two people who buy a lot together are co-owners whether or not they discussed it. The Code assumes as much when it adds that in default of contracts, or of special provisions, co-ownership shall be governed by the provisions of this Title — the rules are a default that fills the silence. This is why families are often startled to learn they have been co-owners for a decade without ever having agreed to be.
What a share is, and what it is not
A one-third share is not the front third of the land. It is a proportion of the whole, which is why a co-owner may sell or mortgage his share freely but cannot convey a specific portion as though it were already his, and why one of them cannot fence off an area and treat it as separate property. Long occupation of a particular part changes nothing by itself; a co-owner in possession is presumed to hold for everyone. Turning shares into parcels requires partition, and until that happens the arithmetic stays abstract.
Working out whether you are in one
Look at the title first. If the certificate names several people without dividing the area between them, you are co-owners. If the property was inherited and no settlement has been registered, you are co-owners even though the title may still be in a dead relative's name. Then look for anything that has already altered the default — a settlement, a partition agreement, a court order, a deed of sale of somebody's share. Bring those documents together before any decision about selling, developing or dividing, because who has to sign is determined by them and not by who has been living there.
Cases citing this provision
These Supreme Court decisions cite the provision above. We list them so you can read them yourself; the summaries of what each decided are not ours to give.
- Sps. Serafin Si vs. Court of Appeals, G.R. No. 122047, October 12, 2000 — read the decision on LawPhil →
- Sps. Salvador and Leonida M. Bangug and Sps. Venerandy Adolfo and Jesusa Adolfo, G.R. No. 259061, August 15, 2022 — read the decision on LawPhil →
- Guillerma S. Silva vs. Conchita S. Lo, G.R. No. 206667, June 23, 2021 — read the decision on LawPhil →
- Heirs of Ernesto Morales, namely: Rosario M. Dangsalan, et al. vs. Astrid Morales Agustin, G.R. No. 224849, June 6, 2018 — read the decision on LawPhil →