HOA not allowing ADUs (California)

[Derrick Dill]

I've been researching townhomes to possibly purchase and want to check in with them regarding if they can build ADUs, if I can convert them and I've got push-back from HOAs stating ADUs aren't allowed (California). AB 670 (New California law passed in August 2019) states: 
"any provision of a governing document, that effectively
prohibits or unreasonably restricts the construction or use of an accessory
dwelling unit or junior accessory dwelling unit on a lot zoned for
single-family residential use that meets the above-described minimum
standards established for those units."

How can I let these HOAs know that they're illegally prohibiting ADUs..? Wasn't looking to get an attorney involved for HOAs to follow the law, but I don't see other options.

Any ideas?