Tenant rights in California, Lease agreements (utilities)

I live in a cottage behind a house full of college students. On Friday, May 1st I gave the landlord the rent and attached was a letter regarding the utitlity bill and why I was being charged $50.00 extra a month. My landlord just gave me a new rental agreement on Monday, May 4th with a line added for $50.00 extra to pay for utilities. Is this legal? (must I add this was never added on the first rental agreement back in November 2008) For the utilities, especially in a college dorm setting, does that landlord divide the total monthly per student renting out a room? If so, is this done in writing or can this be added to the rental agreement?