Short answer. You may sell, assign or mortgage your own undivided share, and even let someone else enjoy it, without the other co-owners' consent. What you cannot do is dispose of any specific portion of the property, because the buyer's right is limited to whatever is allotted to you on division.
What the law says
Each co-owner shall have the full ownership of his part and of the fruits and benefits pertaining thereto, and he may therefore alienate, assign or mortgage it, and even substitute another person in its enjoyment, except when personal rights are involved.
Civil Code, Article 493 — What a Co-owner May Do With Their Share. Read the full provision →
What the law says
the effect of the alienation or the mortgage, with respect to the co-owners, shall be limited to the portion which may be allotted to him in the division upon the termination of the co-ownership
Civil Code, Article 493 — What a Co-owner May Do With Their Share. Read the full provision →
Your share is genuinely yours
Article 493 opens with a wide grant: each co-owner shall have the full ownership of his part and of the fruits and benefits pertaining thereto, and he may therefore alienate, assign or mortgage it, and even substitute another person in its enjoyment, except when personal rights are involved. Read that literally. You do not need permission, a family meeting, or the signatures of your siblings to sell your one-fourth of an inherited lot or to put it up as security for a loan. You are also entitled to your proportionate share of what the property earns — rent, harvests, income — and a co-owner who has been collecting all of it must account to you for your part.
The limit: you own a fraction, not a corner
The second sentence is the one that decides most disputes: the effect of the alienation or the mortgage, with respect to the co-owners, shall be limited to the portion which may be allotted to him in the division upon the termination of the co-ownership. Before partition, no co-owner owns any identified piece of ground. Your interest floats over the whole property as an abstract fraction. So you cannot validly sell "the front 200 square metres facing the road", point your buyer to a boundary, and treat the matter as settled. Your buyer steps into your fraction and must wait for partition, like you, to learn which physical part answers to it.
What the buyer or mortgagee actually gets
A person who buys your share does not become owner of a defined area; he becomes a co-owner in your place, with the same fraction, the same right to the fruits, and the same right to demand partition. A bank that takes your share as security can foreclose only on that fraction. This is why sales of undivided shares are priced cautiously and why lenders often decline them: the security is real but its physical location is unknown until division. The exception in the article — except when personal rights are involved — also matters, because some entitlements attached to a co-ownership are personal to the co-owner and cannot simply be handed to a stranger.
What this article does not give you, and the practical steps
Article 493 gives you power over your share alone. It does not let you sell the whole property, lease out the entire lot, mortgage more than your fraction, or evict a co-owner from land you all hold. A sale by one co-owner of the entire property is not automatically void, but it binds only that seller's share. The article also does not compel partition — that right comes from other provisions — and it does not settle what happens to improvements one co-owner built. Practically: know your exact fraction from the title or the settlement document before you deal with it, disclose the co-ownership to your buyer in writing so the sale is not later attacked as deceptive, register the transaction so third parties are on notice, and take the title and any extrajudicial settlement to a lawyer first, because an undivided share sold on wrong assumptions is expensive to unwind.