If you are paying for your own water, then NO, the building management cannot tap into YOUR supply line and use water for another apartment, or anything else. Generally, when renting, the water is included in the rent. Are you SURE you are paying your own water bill, because if it is indeed included, then you DON'T have a leg to stand on. Of course the building manager acted dumbfounded that you were making a big deal out it, and that is because she KNOWS it's illegal for her to have authorized this if in fact you pay water separately from the rent. This would be akin to the management company coming in and splicing into your cable, electric, or phone line. I would (A) Get a locking cover for my outdoor faucet (they have them), or turn off the water to that particular faucet from inside your unit, and (B) send a CERTIFIED LETTER to the building management stating that you will be taking whatever the additional cost of water was off your next monthly payment (include a copy of your water bill and perhaps the bill before that to show the difference). BTW: Watch out for answers here becaus emany have no clue as to what they are babbling about. Notic ethe one poster who stated: It's not illegal but unlawful. That is the same thing. They start typing and their brains (if they have them) shut down.