Here's a link to the landlord/tenant law: http://www.dca.ca.gov/publications/landlordbook/index.shtml

You need to check your Rental/Lease Agreement. In most (if not all) Rental or Lease Agreement has a section under "No Assignment or Subletting:. ...."Anyone other than the authorized Tenants listed above who resides at the premises more than three (3) days in any thirty day period shall be considered unauthorized sub-Tenant and an incurable violation of the Rental Agreement. If the Landlord should consent to one or more assignment in writing, such consent shall not be deemed a consent to any subsequent assignment or subletting. The acceptance by the Landlord of rent or any other payment from the Tenant or from any other person or entity after a purported assignment or subletting shall not be deemed a consent or waiver by the Landlord to an assignment or subletting, nor a consent or waiver of any subsequent assignment or subletting".

It really depends on what your lease says. You should still have a lease even if you are only renting a room. It does cost the owner more to have an additional person living there but I agree with you, that it should have been in writing from the beginning.