Not to mention a violation of the fire code. Knowingly having more occupants in a room than it is rated for is not a smart thing.People have tried to sneak in without paying for added family and some get away with it while others have been caught. You are taking a big chance. It's called violating resort policy and can get you trespassed. The resort can be held liable for being overly occupied. It's unethical and a form of stealing from Disney. Some people may justify that by saying, Its Disney and we are already paying a high price for that room and they have plenty of $$ and it won't hurt having just one more person. If you booked a room for x number of people, then pay for those people. If it's too expensive then save more $$ before you book.
It doesn't matter that that one person isn't using guest perks. They are utilizing water for showers and bathroom use. Shampoo, soap, towels, toilet paper etc. They are adding more to the cleaning of the room. So they are taking revenue not being paid for by being in that room.
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