In July 2016, California passed AB 857. This legislation requires all completed firearms to have a serial number applied by Jan 1, 2019. An 80% lower receiver is not a firearm, so a serial number is only required once the 80% lower receiver is completed.

If you build an 80% lower into a 100% lower receiver after July 1, 2018, you must FIRST apply to the California DOJ for a serial number, pay a fee, and they will then assign a serial number that you must apply to your firearm. If you built your firearm prior to July 2018 and already engraved a serial number of your choosing that is compliant with ATF regulations, the text of the law seems to indicate that this should be in compliance. However, we have heard that CA DOJ has made statements suggesting that people must engrave a new DOJ issued serial number on their lower even though the lower receiver already has a ATF compliant serial number engraved on it. In our personal opinion, it doesn't seem like this is what AB 857 actually requires, however we are not lawyers and can't offer legal advice.  I would highly suggest you read the law yourself and come to your own conclusion, but if you want to minimize your risk of encountering problems with the CA DOJ, it may be advisable to engrave a CA DOJ issued serial number.