Legal Question in Real Estate Law in California

No. Unless you made the mistake of taking title as joint tenants with right of survivorship, in which case you need to talk to a lawyer directly, your contribution of the down payment entitles you to a larger share of the proceeds.

If you paid 100% of the down-payment and did not make a gift of some ownership to your ex, you would be considered the 100% equitable owner of the home, irrespective of how "legal" title were taken and is now held. This also assumes no contrary contract between you. The principle is called "purchase-money resulting trust" and is based upon an equitable presumption that title was intended to follow down-payment unless there is also a legal presumption of a gift arising from close relationship (husband-wife, parent-child). Boyfriend-girlfriend does not create such a presumption, however.