In the Philippine legal landscape, the union of two individuals—whether through the solemnity of marriage or the informal nature of cohabitation—carries significant implications for property ownership, inheritance, and financial liability. Under the Family Code of the Philippines (Executive Order No. 209), the law provides a default framework for these relations, but parties have the liberty to define their own terms through written agreements.

I. Prenuptial Agreements (Marriage Settlements)

A prenuptial agreement, legally referred to as a Marriage Settlement, is a contract entered into by future spouses to fix the property regime that will govern their marriage.

1. Legal Requirements for Validity

For a prenuptial agreement to be valid and enforceable under Philippine law, specifically Article 77 of the Family Code, it must meet the following criteria:

Timing: It must be executed before the celebration of the marriage. Any agreement made after the wedding is generally void, unless it is a judicially approved separation of property.
Form: It must be in writing. Oral agreements regarding property settlements are not recognized.
Signatures: It must be signed by both intended spouses.
Capacity: The parties must have the legal capacity to enter into a contract. If a party is a minor (under the age of 21, in the context of parental consent for marriage settlements), the person authorized to give consent to the marriage must also sign the agreement.

2. Registration for Third-Party Bindery

While a signed written agreement binds the husband and wife, it does not affect third parties (such as creditors) unless it is:

Notarized; and
Registered in the Local Civil Registry where the marriage is recorded and in the relevant Registry of Deeds.

3. Choosing a Property Regime

In the absence of a prenup, the law imposes the Absolute Community of Property (ACP) for marriages celebrated after August 3, 1988. Spouses may use a prenup to opt for:

Conjugal Partnership of Gains (CPG): Only the fruits/income of individual properties and those acquired during marriage are shared.
Complete Separation of Property: Each spouse retains ownership, management, and enjoyment of their respective past and future properties.
Mixed Regimes: A combination of systems, provided they do not violate law or public policy.

II. Cohabitation Agreements (Common-Law Relationships)

For couples living together without the benefit of marriage, property relations are governed by Articles 147 and 148 of the Family Code. A cohabitation agreement acts as a private contract to clarify these statutory defaults.

1. Unions Without Legal Impediment (Article 147)

This applies to couples who are both legally capacitated to marry each other but choose not to.

Default Rule: Property acquired through joint efforts is owned in common (50/50).
The "Housewife/Househusband" Provision: Even if one party did not contribute financially, their care and maintenance of the family home are legally deemed a contribution to the acquisition of property.

2. Unions With Legal Impediment (Article 148)

This applies to adulterous or bigamous relationships, or those not capacitated to marry.

Default Rule: Only properties proved to have been acquired through actual joint contribution of money or industry are co-owned. There is no presumption of equal sharing.

3. Utility of a Written Agreement

While the term "Marriage Settlement" does not apply to non-married couples, they may execute a Contract of Co-ownership. This agreement can:

Inventory properties brought into the relationship.
Define the percentage of ownership based on actual financial contribution.
Outline the process for liquidating assets should the cohabitation end.

III. Key Benefits of Formal Agreements

Clarity and Conflict Prevention: By establishing a clear roadmap of "who owns what," couples can avoid the emotional and financial drain of litigation during a separation or upon the death of a partner.

1. Protection of Pre-marital Assets

For individuals entering a union with significant inherited wealth or established businesses, a prenup ensures these assets remain separate and are not "absorbed" into a community pool.

2. Insulation from Debts

Under Absolute Community of Property, the common fund may be liable for debts contracted by one spouse if they redounded to the benefit of the family. A separation of property regime can protect one spouse's assets from the financial liabilities or business risks of the other.

3. Certainty in Succession

Upon the death of a spouse, the liquidation of the property regime is the first step before the distribution of the estate. A prenup simplifies this process, making it easier for heirs to identify the deceased's actual estate.

IV. Prohibited Provisions

A prenuptial or cohabitation agreement is not a "blank check." Certain stipulations are considered void as they contravene Philippine law or public policy:

Waiver of Support: Spouses cannot validly waive the legal obligation to provide mutual support.
Derogation of Authority: Provisions that strip a spouse of their legal right to exercise parental authority or manage their own civil status are invalid.
Illegal Acts: Any clause encouraging or stipulating for the commission of a crime or immoral act.
Pre-determined Custody: Agreements on future child custody are generally not binding on courts, as custody is determined by the "Best Interests of the Child" at the time of the dispute.

Summary Table: Property Defaults

Relationship Type	Legal Basis	Default Regime (No Agreement)
Married (after 1988)	Art. 75, Family Code	Absolute Community of Property
Cohabiting (Capacitated)	Art. 147, Family Code	Co-ownership (Presumed Equal)
Cohabiting (Incapacitated)	Art. 148, Family Code	Co-ownership (Proven Contribution Only)

In the Philippine context, these agreements are not signs of "planning for failure," but rather sophisticated tools for financial planning and the protection of individual rights within a partnership.