Yeah, I know, right? And we have homeowner's insurance for that! I wanted to buy it, pour ~10K into it, and resell it. I pay 575/month in rent currently, so the savings would be nice.
Depending on how handy you are, you can get away with being at or under budget. Just have a realistic budget that accounts for those unknowns you'll find when tearing the house apart.
Perfect example was when we were remodeling a relative's house. We had completed the rest of the house and were just about done with the kitchen. I had finished redoing all the messed up drywall and we had finished painting. It was the day before the cabinet guys were coming and my wife happened to check what was supposed to be a half hot, for the dishwasher/disposal. Turned out the whole socket was on the switch(there was no dishwasher, so no one ever knew). I had to tear out part of the wall from the socket to the switch and rewire it correctly. Luckily it was summertime and warm, so I was able to redo the drywall and have the mud dry by the end of the day to paint.
Not a budget buster at all, but still one of those things that you expect to work correctly.