Landlord raised rent by $400. Is that legal?

Speaking generally — in California, the statewide AB 1482 caps annual rent increases on covered units at 5% + local CPI (up to 10% max), and the landlord must give 30 days notice for increases under 10% and 90 days for larger ones. BUT AB 1482 doesn't cover everything: single-family homes (unless owned by a corporation/LLC), units built in the last 15 years, and owner-occupied duplexes are exempt. Many cities (LA, SF, Oakland, San Jose, Berkeley) have local rent-control ordinances with lower caps that take precedence. Look up your specific city's rent board. A $400 increase on a $1,500 base would be ~27% — way over any cap I know of.

Tenants Together in CA has a hotline that helped me confirm my increase was illegal. They walked me through how to write the response letter.