While renting an apartment, the owners sold the property. Can the new owners subject us to new rates right away? They also hired a billing provider whose service costs were added to our bill.

What's the legal thing to do? Thanks

You are still living in the premises under the terms of the written lease between you and the former landlord. If the new owners wants to change those terms then they have to send you the appropriate notices terminating the old lease and giving you the terms of the new lease. If I were in your position I would not pay any increased rates or billing costs. Speak to an attorney in your area to make sure of your rights. Most attorneys will offer a free consultation. You can also contact one of the agencies listed at http://www.dca.ca.gov/legal/landlordbook/appendix3.htm to get more information.