It is most likely legal pursuant to Civil Code §1366.1. The reason I say most likely is because the late fees could be challenged if the monthly assessments for this particular HOA is extraordinarily high (such as over $1,000 per month), then 10% might be considered excessive if the HOA's actual costs for collection are not that much.
Frank W. Chen has been licensed to practice law in California since 1988. The information presented here is general in nature and is not intended, nor should be construed, as legal advice for a particular case. This posting does not create any attorney-client relationship with the author. For specific advice about your particular situation, consult with your own attorney.