The affects of destroying your interest vary depending on whether you are in a TIC or a JTWROS. Without a co-ownership agreement, in a TIC, the tenant wishing to destroy his or her interest may obtain a partition of the property. A partition of the property divides any land into distinctly owned lots. Sometimes, especially with a house, this is not possible. In that case, a forced sale of the property could be conducted, with the proceeds being divided according to shares. Each co-owner is entitled to the right of a court-ordered partition. The good thing about determining who owns what percentage ahead of time in a co-ownership agreement is that you can avoid the court’s interference in partitioning. In your agreement you can also waive the right of partition.
When a JTWROS tenant terminates his or her interest, the remaining co-owners keep their JTWROS between them and remain joint owners of the remaining interest. If the terminating tenant conveys his or her property to a third party, however, that third party owns his or her share on a TIC basis with the other tenants. The original tenants still preserve their joint tenancy interest between each other, while the new tenant is a tenant in common with the other two.
This result arises because the timing is different. The original tenants all received their interest in the home at the same time, whereas the new tenant received his or her interest at a later time. If all the tenants wish to maintain a joint tenancy, then all of the original tenants must transfer the joint interest of the remaining joint tenants and the new joint tenant together, in one instrument. Absent an agreement that specifies otherwise, this is what happens when a tenant breaks or destroys his or her interest.
One way around the default approach is to actually specify in the co-ownership agreement that a selling co-owner must preserve an opportunity for the remaining tenants to purchase the interest before any third party. Adding this provision makes sense; however, you must also think about how you will fairly assess the property value at that time, whether the remaining co-owner must accept the sale offer, and what will happen if the remaining co-owner does not have sufficient funds to accept the sale offer.
Buying a house together has its perks, as long as all the parties involved are thoughtful and careful in deciding what will work best for each of them. Often times, it is a good idea for each of them to consult an attorney who will look out for their individual property interest.