Whether your home is twenty or one hundred & twenty years old, it is quite likely that your remodeler will encounter some unforeseeable condition—the older the house the more likely. It is advisable to have a contingency fund for such instances; ten-percent of the project cost is not unreasonable. If you don’t spend it, you have funds for accessorizing your newly remodeled space.
While experienced remodeling contractors can anticipate a likelihood for some conditions, it is unreasonable to expect them to do any more than advise you beforehand. If a remodeler were to budget for every imaginable unforeseeable condition, and the very definition of unforeseeable precludes him/her from doing so, one of two scenarios would play out.
If competitively bidding, including even a reasonable presence of unforeseeable conditions would likely price their proposal out of competition with others who do not include them in their bids, and it is the norm not to include them.
Secondly, if a remodeler did include contingency funds for unforeseeable conditions that did not materialize, the homeowner ends up paying for work that isn’t needed.
Examples of unforeseeable conditions include termite or water damage to framing, a common but relatively easy fix. Another is in kitchens w/furr downs, aka soffits, above the kitchen cabinets just below the ceiling; plumbers and electricians often run their pipe or cables through the furr downs rather than straight up through the wall. If removing furr downs as part of the kitchen remodel, these plumbing lines and electrical cables will have to be re-routed. Your remodeler does not have x-ray vision and can only guess their presence.
A bigger unforeseeable condition can occur when an upstairs bath in a two-story house, has a plumbing drain coming down through a wall that you wish to remove to open up rooms on the 2nd floor. If the bathroom is directly over the wall, you may anticipate it is there, but that is not always the case.
A recent project, the conversion of a 100 year-old drug store to a single-family home, resulted in having to completely re-frame from the inside, an exterior wall that had fire damage, fractured wall studs AND wood siding facing the inside of the soon-to-be kitchen.
I suggest you ask any remodelers you are discussing your project with what type of unforeseeable conditions they think you may encounter and what the cost may be to correct; just do not ask them to include it in their proposal.