'Per Additional Person' Fee where All Utilities are Included in Rent (CA)

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just checking, does anyone know if this is legal to do? am in Los Angeles county and noticed where 1 or 2 tenants sign lease but have ppl often visiting/living with them, the electric & water can really shoot up from one bill to the next! 

am thinking of doing $100 per additional person to cover the additional utilities (this is socal, water and electric are quite pricey!) 

adding meters to the current 2-family property in question isn't an option cause of crossed water and electric lines between 1st and 2nd stories, and a single water heater.

also wondering how long of a stay do landlords allow before a guest is considered a occupant? i have seen quite a few ads where owner stipulates NO overnite guests at all but dont know if i wanna be thaaat strict... mehhh