Personally, no matter how beautiful their dam is, if you enter their property - even if the gate is open - then my understanding is that you'd be on very shaky ground, and more than likely trespassing. If you're on the side of the road taking photos, even with their house IN the frame, my understanding is that you would be on much firmer ground, but I do understand your reticence, as I'd feel very uncomfortable with that scenario without their permission. I imagine a polite approach and request would generally be met favourably by most reasonable people. They probably agree that their property is photogenic anyway, and would probably love a copy of any image you took.