Obviously having it in a contract is super important. I think also having a conversation about it, in some cases multiple times, and give tell them what your process is when you discover those things. Look at it from their perspective, they areinvesting tens of thousands of dollars and once you demo, they are in a vulnerable position. If you already described what will happen ahead of time, then back it up with evidence that makes sense to them, and also give them options, it will be a much easier pill to swallow. I like to give them other examples of times when that happened, what the cost was, and how we handled it.
If they are super concerned about it, they can pay you to do exploratory work. if that isn't an option, refer to the process above.
