Legal Question in Real Estate Law in California

Yes, of course; all it takes to "sue back" (cross-complain or counterclaim) is to write up the cross-complaint, take it to the courthouse, and pay a modest filing fee. Of course, your real question is, or should be, "Will such a cross-complaint be successful?" This depends upon a couple of things, including an adequate legal presentation, but mainly upon the quality of their facts. So, the real question is whether there are facts present, which if expressed in a lawsuit with reasonable skill, might convince a judge or jury that these erstwhile tenants actually do have a valid legal claim to ownership. Now, ownership of real property is generally initially established by a visit to the county recorder's office and a careful review of recorded documents, mostly deeds. However, other kinds of recorded documents may also come into play, and occasionally ownership claims can be pleaded successfully upon facts that don't appear in the public record. Overall, if you suspect that your "tenants" might claim ownership, it is very much worth your time and a small fee payment (if demanded) to have an initial consultation and basic research undertaken by an experienced real estate lawyer. Further, if you are served with a cross-complaint, seek professional legal assistance immediately!