Detailed Answer

Short answer: In Washington, the petitioners in a partition lawsuit are the person or people who bring the suit — typically one or more legal owners of the property (co-owners such as tenants in common or joint tenants, a trustee, or a personal representative). You do not need every interested party to join as a petitioner for the court to order partition, but the court usually requires that all persons with a legal or equitable interest in the property (owners, heirs, trustees, lienholders, mortgagees, persons in possession, minors, incapacitated persons, and others claiming an interest) be joined as parties so the court can resolve all interests in the single proceeding. For Washington statutory authority on partition, see RCW Chapter 7.60: https://app.leg.wa.gov/rcw/title-7/chapter-60/.

Who can be a petitioner?

Any co-owner of record who seeks to divide or sell the property (for example, one tenant in common who wants the property sold can file alone).
A group of co-owners who agree to pursue partition together.
The trustee of a trust that holds title to the property (acting on behalf of the trust).
The personal representative or executor of a decedent’s estate when the estate owns the property.
A guardian or conservator who has authority over property owned by a minor or incapacitated person, where the guardian/conservator brings the action on behalf of that person.

Who must be named in the lawsuit (so the court can fully resolve ownership)?

Washington courts require joinder of all persons who have or claim an interest in the property so the court can adjudicate competing claims. That generally includes:

All recorded owners (and those who claim an ownership interest).
Spouses and community-property co-owners when marital/community interests exist.
Trusts, trustees, and beneficiaries if title is in a trust.
Personal representatives, heirs, and devisees if an owner is deceased.
Minors or legally incapacitated people — and their guardians, conservators, or next friends.
Mortgagees and lienholders (they are usually joined as defendants so the court can determine lien priority and order sale or allocation of proceeds).
Persons in actual possession (tenants or adverse possessors) and anyone who claims an easement or other encumbrance.
Unknown or unlocatable claimants (often sued as “unknown heirs or persons claiming any interest”) so the court’s decree binds or extinguishes their claims.

How to decide whether to list someone as a petitioner or a defendant

Petitioner = the party bringing the action. Defendant = a person who holds an adverse or competing interest and is joined so the court can clear title and distribute proceeds.

Common patterns:

If you and other co-owners all agree to a partition or sale, you can file the petition together as multiple petitioners. Doing so can simplify notice and avoid fighting about costs.
If one co-owner wants partition but others oppose or are uncooperative, the requesting co-owner files alone as petitioner and names the others as defendants. The court can proceed without their voluntary participation after proper service.
Creditors, mortgagees, and lienholders are usually defendants because the court must determine their claims against the property and how sale proceeds will be distributed.

Special situations

Deceased owner: If an owner died and the property passed by will or intestacy, the petitioner should name the estate’s personal representative or the known heirs/devisees. If heirs are unknown, include “unknown heirs” defendants and follow local practice for service or publication.
Minor or incapacitated owner: The petitioner should name the child or incapacitated person and also join their guardian/conservator, and the court may require a guardian ad litem to protect that person’s interests.
Trust-owned property: The trustee typically acts for the trust and should be the petitioner (or a defendant if someone sues the trust). Beneficiaries who claim an interest should be named so the court can resolve disputes.
Multiple security interests: Mortgagees and judgment lienholders must be named so the court can determine priorities and satisfy liens from sale proceeds.

Practical steps to determine who to include

Run a title search at the county recorder to list all recorded owners and encumbrances.
Check probate records for any deceased owners and for appointed personal representatives.
Check the county assessor and tax records, and search UCC filings for unrecorded liens.
Identify occupants and contract purchasers or anyone claiming a possessory interest.
When in doubt, name the person or entity and describe the claimed interest. The court will sort priorities once parties are served.

Where the law says so

Washington’s statutes and case law govern partition procedure and joinder of parties. For statutory guidance, see Washington’s partition statutes in RCW Chapter 7.60: https://app.leg.wa.gov/rcw/title-7/chapter-60/. Those provisions explain the remedies available and how the court may divide or order sale of property and distribute proceeds among claimants.

Example hypotheticals

Hypothetical A: Three siblings hold property as tenants in common. One sibling wants a sale. That sibling files the partition petition alone as petitioner and names the other two siblings as defendants. The petition must also name the mortgage lender as a defendant so the court can address the mortgage lien.

Hypothetical B: Property titled in a trust. The trustee joins the petition as petitioner on behalf of the trust. Beneficiaries who claim income or distribution rights should be named so the court can resolve any competing claims.

Next steps and recommended practices

Do a thorough title and public-records search before filing.
If minors or incapacitated persons are involved, anticipate appointment of a guardian ad litem.
Consider negotiation or mediation with co-owners before filing — litigation is expensive.
If you plan to file, prepare to join all known claimants and encumbrancers to avoid later collateral attacks on the court’s decree.

Helpful Hints

Start with a title search at the county recorder’s office to find all owners and lienholders.
Name the party who brings the action as the petitioner; name everyone else who claims an interest as defendants so the court can dispose of all claims.
If multiple owners agree, join them all as petitioners to show unity and reduce contested issues.
For deceased owners, locate the personal representative or list heirs; if unknown, include “unknown heirs” and follow court rules for notice.
If a minor or an incapacitated person has an interest, expect the court to require a guardian ad litem or court approval of settlements or sales that affect that person.
Include mortgagees and lienholders as defendants so the court can order a sale free and clear of their claims or allocate proceeds to satisfy liens.
Talk to a Washington real property attorney or the clerk of your local Superior Court if you are unsure who to name — early advice can avoid mistakes that delay the case.

Disclaimer: This article explains general Washington law about partition actions and is for informational purposes only. It does not constitute legal advice, create an attorney-client relationship, or predict outcomes in any particular case. For advice tailored to your situation, consult a licensed Washington attorney.