The application of the doctrine of laches requires the passage of time combined with a change in condition which would make it inequitable to enforce the claim against the defendant. The defendant must prove a lack of due diligence on the part of the plaintiff which resulted in some prejudice to the defendant. The concept of "due diligence" on the part of the plaintiff encompasses the situation in which the plaintiff does not know of the condition giving rise to the claim (in your example, the fence) for a period of time... the question is whether the plaintiff knew or should have known of the condition.