Legal Question in Real Estate Law in California

Either can force sale through a partition action, unless there is a writing to the contrary.

When two or more people co-own real estate as tenants in common or joint tenants, or sometimes as partners, any one of the co-owners, if unhappy with the co-ownership, can bring a special kind of lawsuit called a "partition" suit to force division of the property. Historically, this was done by dividing the parcel into smaller parcels and giving one to each (former) co-owner. Nowadays, with property increasingly urbanized and subject to zoning laws and policies, partition is almost always accomplished by a court-ordered and -supervised sale of the property, followed by a division of the net proceeds after paying off loans and liens.

Partition is always available to a co-owner unless there has been an express or implied waiver of the right to partition.

Generally, disability of a co-owner is not a waiver, so either the 25% owner or the 75% owner should be able to ask the county superior court to order partition by sale through the preparation, filing and service of an appropriate lawsuit.

Many partition cases are filed and then promptly settled out-of-court when the defendant(s) realize the inevitable result and decide to save the costs of litigation by agreeing to a private sale. If there are issues that stand in the way of an out-of-court settlement such as who should pay for repairs, etc., these can be submitted for private arbitration upon agreement of the co-owners to do so.

To supplement my previous answer: The usual result of a partition by sale is that a third party ends up owning the property. Partition is not a process whereby the majority owner forces out the minority owner so as to end up as the sole owner. That may sometimes happen, because nothing precludes a current owner from bidding, thus becoming both the buyer and a partial seller in the same auction. However, within the partition law (Code of Civil Procedure sections 872.010 et seq.) there is a procedure called "partition by appraisal" that allows one co-owner to buy out the other at an appraised price when all co-owners agree to the process. In you case, though, would the 25% owner agree? Sounds doubtful.