California counties will not automatically reassess homes due to the recent fires, because the law requires that the counties first receive an application from the homeowner. Those who lost property will need to file the appropriate county casualty abatement form for the 2016-2017 year.
Many property tax bills for the 2016-2017 were just mailed out over the past week by various counties. People will need to be proactive about reaching out to the county if they currently have no home at which to receive their mail.
Here is the link to the County of Sonoma, which has a link to the Calamity Damage Reassessment form on the front page: http://sonomacounty.ca.gov/CRA/Assessor/
Following the President and FEMA's declaration that a major disaster exists in the State of California, the IRS announced that California residents of many counties will receive tax relief.
Deadlines for many taxpayers who had a valid extension on their 2016 and other tax returns due in October, will be extended to January 2018. The announcement included a reminder to taxpayers about how to properly claim casualty losses.
If you were affected by the recent California wildfires, click here to learn more about federal tax relief that may be available to you: https://www.irs.gov/newsroom/tax-relief-for-victims-of-wildfires-in-california