Cal. Gov. Code § 50555

At any time not later than the time set for hearing objections, any owner of property assessed or being assessed to pay for the acquisition, construction or installation of the improvement proposed to be sold may make and deliver to the clerk of the legislative body a written protest against the proposed sale. Failure to object in writing at that time constitutes a waiver of all objections to the proposed sale and a bar to any subsequent attack on the validity of such sale.