Ah, wonderful Contra Costa County... my original home (was raised in Concord).....
From the California Department of Consumer Affairs:
"How often can rent be raised?
If you have a lease, your rent cannot be increased during the term of the lease, unless the lease allows rent increases.
If you have a periodic rental agreement, your landlord can increase your rent, but the landlord must give you proper advance notice in writing. The written notice tells you how much the increased rent is and when the increase goes into effect.
The amount of advance notice must be at least as long as the period of time between your rent payments, unless you've agreed to a shorter notice
period in your rental agreement. For example, in a month-to-month tenancy, the landlord must give you at least 30 days' advance written notice before
the rent increase takes effect (unless you've agreed to a shorter notice period). If you pay rent every week, a seven-day advance written notice is required. A notice of rent increase must always be at least seven days.
Normally in the case of a periodic rental agreement, the landlord can increase the rent as often as the landlord likes. However, the landlord must give proper advance notice of the increase, and the increase cannot be retaliatory. Local rent control ordinances may impose additional requirements on the landlord.
Increases in rent for government-financed housing usually are restricted. If you live in government-financed housing, check with the local public housing authority to find out whether there are any restrictions on rent increases.
Landlord's notice of rent increase
A landlord's notice of a rent increase (or any change in the terms of the rental agreement) must be in writing and must be served by one of the methods described in "Proper Service of Notices."
Most notices of rent increase state that the increase will go into effect at the beginning of the rental period. For example, a landlord who wishes to increase the rent on a month-to-month rental effective on October 1 must see to it that the tenant receives the notice by September 1. However, a landlord can make the increase effective at any time in the month if proper advance notice is given.
If the increase in the rent becomes effective in the middle of the rental period, the landlord is entitled to receive the increased rent for only the last half of the rental period."
Source:
http://www.dca.ca.gov/legal/landlordbook/livingin.htm