answer:Cut them back to what height? If the part cut was < 50% they have a good chance to survive I think. So, if they are still 8+ feet high you have a good shot at them doing ok. Are they a solid mass of bush to the ground? Not a tree right? How big are your properties? One acre? Ten? Are there covenants or any deed restrictions in your neighborhood that dictate anything about plants and lawns? If they are actually bushes, full to the ground, lined close together, I would be pretty upset if my neighbor had theirs 15 feet high, especially if our properties are small or if it is close to my house. However, cutting them behind your back seems pretty awful to me. You say on the line, meaning your side of the line?