Another heir sold their share of inherited co-owned property to a buyer—how to respond to an unknown co-owner, and the exits

The key point: In co-owned property, each co-owner may in principle sell their own share to a third party without the other co-owners' consent. If an unknown co-owner (such as a share-buying business) contacts you, first confirm their position and what they want, and do not reply on impulse. Compare the exits—selling your share, buying theirs, or selling the whole property—by net proceeds.

You left the family home or land you inherited with your siblings in joint names, and one day a notice arrives: "We have become your co-owner," "We will buy your share," or "We demand partition of the co-owned property." This is the case where another heir has sold their co-ownership share to a share-buying business or a third party.

If, startled by the sudden contact, you sign as told—or conversely keep ignoring it—the situation can turn against you. The starting point is to organise the system and the exits.

Yotsuba Real Estate Co., Ltd. (四葉不動産株式会社), based in Bunkyo, Tokyo, provides consultations and brokerage for selling inherited property and co-ownership shares. We can start, from the seller's position, by estimating the market value of the share and comparing the options of selling, buying, or selling the whole.

Can another heir sell their co-ownership share to a third party without permission?

The conclusion is yes, they can. For co-owned property, each co-owner may dispose of their own share alone and freely, without the other co-owners' consent. This follows from the view that a co-ownership share, like ownership, is an object of disposition, and it is established in case law.

On the other hand, selling or substantially altering the "co-owned thing" itself (the whole property) requires, in principle, the consent of all co-owners. The Civil Code divides the roles: a change to the co-owned thing (article 251) requires unanimous consent; matters of management (article 252) require a majority by share value; and acts of preservation may be done by each co-owner alone.

What you can do	Consent required
Sell your own co-ownership share to a third party	No consent of the other co-owners (possible alone)
Sell the whole property (the co-owned thing)	Consent of all co-owners in principle
Manage the co-owned thing (short-term lease, etc.)	Majority by share value

In other words, the fact that another heir sold their share usually cannot be prevented in law. For selling the whole co-owned property with everyone's consent, see Selling inherited co-owned property with everyone's consent. This article focuses on the response "after" a third party has acquired a share and become a co-owner.

When an unknown co-owner (a buyer) contacts you, what should you confirm first?

When a notice arrives, do not reply on impulse; confirm the facts. Verify, through documents and the register, whether the other party has really become a co-owner and what they are seeking.

• Obtain a certificate of registered matters and check whether a share-transfer registration has actually been entered, and who holds what fraction
• Distinguish whether the other party seeks "to buy your share", "to sell their share to you", "to sell the whole property", or "partition of the co-owned thing"
• Receive the proposed amount, deadline and conditions in writing, without accepting or rejecting on the spot
• Secure time to organise the content even if a short reply deadline is set

A buyer may use the acquired share as a foothold to offer to buy out the other co-owners, or to demand partition, ultimately aiming to acquire the whole property or convert it to cash. Having grasped the other party's position, calmly compare your exits—sell, buy, or sell the whole. The negotiation itself, and the judgment of whether you can legally resist their demands, are the lawyer's field.

What is a partition claim, and will it be division in kind, conversion to cash, or price compensation?

A co-owner may at any time demand partition of the co-owned thing (Civil Code article 256; however, an agreement not to partition is possible for up to five years). If discussions do not settle, it becomes partition by judgment (Civil Code article 258).

The 2021 amendment to the Civil Code (Act No. 24 of 2021, in force on 1 April 2023) organised the methods of partition by judgment in the statute. Broadly, the order is as follows.

Method of partition	Content (outline)
Division in kind	Dividing the thing itself, such as subdividing the land
Compensation (price compensation)	Some co-owners acquire the others' shares and pay the consideration (market-value equivalent)
Conversion to cash (auction)	Where division in kind or compensation is not possible, etc., auctioning and dividing the proceeds (Civil Code article 258, paragraph 3)

Under the amended article 258, paragraph 2 provides for division in kind and for compensation division in which a co-owner is made to bear an obligation and acquire the others' shares; paragraph 3 allows the court to order an auction where these are not possible or where partition would significantly reduce the value. Because an auction tends to yield lower net proceeds than a voluntary sale on the market, the stage of discussion and negotiation before it matters.

Where inherited co-ownership (estate co-ownership) is involved, the procedure may change in relation to estate division. Which method applies, and which court procedure to use, should be confirmed with a lawyer.

How do net proceeds differ between selling your share, buying theirs, and selling the whole?

There are broadly three exits. Each differs in the amount you receive, the burden of procedure, and the consent required. Compare them including not only the amount but also tax, registration costs and time.

Exit	Overview	Main points
Sell your share	Sell your share to the other party (buyer) or a third party	The market value of a share alone tends to be lower than the share fraction of the whole. Capital-gains tax
Buy their share	Buy the other party's share to resolve the co-ownership and become sole owner	Purchase funds, market valuation of the share, price negotiation with the other party
Sell the whole property	All co-owners sell the whole and divide the proceeds by share	Consent of all is required. If it sells at market price, net proceeds are relatively larger

Selling only a share tends to fetch less than the share fraction of the whole, because buyers are limited. Conversely, if you can buy the other party's share and become sole owner, your freedom to sell or use it in future increases. Which is more favourable depends on the amount, your funding capacity, the other co-owners' intentions, and whether it is urgent.

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Estimating the market value of shares and property, and brokerage of sales and purchases, are handled by the real-estate business (our company). For the valuation of shares and the reasoning of compensation division, see Compensation division, valuation and sale of inherited property; for the practice of selling shares where heirs living overseas, including in China, are involved, see Selling co-ownership shares where there are heirs living overseas. A valuation does not guarantee the sale price or net proceeds.

If you leave the co-ownership as it is, what disadvantages arise in fixed asset tax and management?

"Just waiting and seeing" can let burdens and disadvantages pile up.

• Fixed asset tax and city planning tax are treated as owed jointly and severally by the co-owners; even if billed to a representative, the internal burden by share becomes an issue
• Because decisions on repairs and management are made by a majority of shares, decision-making is harder to progress when an unknown co-owner is present
• The other party (buyer) may demand partition, leading to an auction with lower net proceeds
• If further inheritance occurs, the number of co-owners increases and relationships grow complex

Inheritance registration has been mandatory since 1 April 2024, and failing to apply within the deadline without justifiable grounds can be subject to an administrative fine. Leaving co-ownership as it is overlaps with such registration and tax problems. Leaving it is not always disadvantageous, but decide whether to act or wait after grasping the substance of the disadvantages. Confirm the specific fixed-asset-tax burden and capital-gains tax with a tax accountant, and registration with a judicial scrivener.

How do you divide roles with a lawyer, judicial scrivener and tax accountant?

Heirs who hold a co-ownership share can contact Yotsuba Real Estate directly to request consultation and brokerage for a sale or purchase. Please first tell us:

• The property's address and approximate share fraction
• The content of the notice received (buying your share, selling theirs, or partition)
• The co-owners and shares confirmed in the certificate of registered matters
• Your preferred exit (sell, buy, or sell the whole) and whether it is urgent

Yotsuba Real Estate organises the estimated market value of the share and property, a comparison of net proceeds by exit, and how to proceed with a sale or purchase. You can also reach us from the consultation desk for selling inherited property.

The roles are divided as follows. Where discussions on partition fail and develop into a partition-claim lawsuit or mediation, and the negotiation with and judgment of legal resistance to the other party, are handled by a lawyer; market valuation and brokerage of the sale or purchase of shares and property by the real-estate business (our company); registration of share transfers and partition by a judicial scrivener; and tax matters such as calculating capital gains and acquisition cost by a tax accountant. These are each independent businesses, and you contract with them separately. We do not receive referral fees.

Consult us about co-ownership shares and co-owned property—initial consultation is free

In your message, write "Inherited co-ownership share" and include the share fraction and an outline of the notice received. We begin with the materials to confirm and the exits to compare, without presuming sell, buy, or leave it.

Frequently asked questions

Q. Isn't it illegal that another heir sold their share without my consent?
A. Because each co-owner may dispose of a share alone and the other co-owners' consent is not required in principle, the sale of a share itself is usually lawful. Whether you can argue it is unlawful or void depends on the individual circumstances, so confirm with a lawyer. Yotsuba Real Estate handles the market value of the share and organising the exits.

Q. What happens if I keep ignoring the buyer's contact?
A. If the other party demands partition and discussions do not settle, it proceeds to a court procedure, and there can be a path to an auction with lower net proceeds. Rather than ignoring, confirming what the other party wants and comparing the exits—sell, buy, or sell the whole—keeps more options open.

Q. Will selling only my share be cheaper than selling the whole?
A. A share alone tends to fetch less than the share fraction of the whole, because buyers are limited. Decide after lining up the exits and net proceeds—buying the other's share, or all co-owners selling the whole.

Q. I want to buy the other party's share. Where do I start?
A. First confirm the shares in the certificate of registered matters, then organise an estimate of the share's market value and prepare purchase funds. Divide the roles—price negotiation with a lawyer, registration with a judicial scrivener, valuation and brokerage with the real-estate business—and proceed. Yotsuba Real Estate organises the valuation estimate and how to proceed.

Sources

The following public materials are starting points for investigation. All were consulted on 17 October 2026. Laws are amended, so check the current text, enforcement date and latest amendment on e-Gov Law Search. They do not establish the validity, net proceeds or tax liability in an individual case. The sources below are in Japanese.

• e-Gov Law Search: Civil Code, arts. 249–262-2 (co-ownership; change, management and partition of the co-owned thing; acquisition of the share of a co-owner whose whereabouts are unknown), art. 256 (partition claim), arts. 258 and 258-2 (partition by judgment), art. 262-3 (transfer of the share of a co-owner whose whereabouts are unknown)
• Ministry of Justice: 2021 amendments to the Civil Code and Real Property Registration Act, and the Act on State Acquisition of Inherited Land: revision of the co-ownership system (Act No. 24 of 2021, in force on 1 April 2023)
• Ministry of Justice: Mandatory application for inheritance registration: the inheritance-registration obligation from 1 April 2024
• National Tax Agency Tax Answer: No. 3105 Assets subject to capital gains and the method of taxation and No. 3202 How to calculate capital gains (separate taxation): checking taxation on the transfer of a co-ownership share

This article provides general information. Individual legal judgments require review by the appropriately qualified professional. It does not guarantee the feasibility, amount, tax, or partition outcome of a share sale.

Joji Uramatsu (浦松 丈二), Representative Director and licensed real estate transaction specialist, Yotsuba Real Estate Co., Ltd. (四葉不動産株式会社). We provide brokerage for selling inherited property and co-ownership shares. Author profile

Updated: 17 October 2026