The provisions of existing laws regarding the service of process shall apply to service in proceedings under this chapter. The person in whose name the real estate affected by the action stands on the books of the tax collector for purposes of taxation shall be presumed to be the owner thereof, and in case of unknown persons having or claiming any ownership, right, title or interest in property affected by the action, such may be made parties to the action by designating them in the summons and complaint as “all other persons unknown claiming any ownership, right, title or interest in the property affected by the action,” and service thereon may be had by publishing in the manner prescribed by law. Any person having or claiming such ownership, right, title or interest, and any owner or agent in behalf of himself and such owner, may make, serve, and file his answer therein twenty (20) days after such service and have trial of his rights in the premises by the court, and if said cause had already proceeded to trial or to findings and judgment, the court shall by order fix the time and place of such further trial and shall modify, add to, or confirm such findings and decrees as the case may require. Other parties to said action shall not be affected thereby.