You have a couple of issues here. The septic would be the first and you should have an inspection of the system by a septic company that has a great reputation. Your septic contingency will probably have to be longer than normal to have the "discovery time", and you may even have a septic designer look at it too. You should also ask to attend that inspection. For a home being 20 years old, it is not uncommon to not have the "As-Built" on file with either the City or the County, depending on where the home is located. The other issue would be the additions and if there were any L & I inspections that should have been done to the home. Without the proper permits, I am guessing that was not done. Contact the County if it is not in the city limits ( which would be Snohomish ) or the City in regard to that. CUZ Septic has always done good work for my clients and there are some great Septic designers out there as well. Absolutely, check with your lender to make sure that the lack of permits will not be an issue. There are also Home Warranties that are good and will cover some of those issues. AHS is one that has a flex plan that will cover pre-existing conditions and that may be something else you might want to consider, or even to have the Seller provide that for you.
Good Luck to you and I hope that I was helpful in my answer.
Natalee Thurston

The challenge is the mortgage company you are getting the loan through. Check with them and see if they will loan you money if the home does not have an approved septic design with the Health District. Depending on the system it may or may not bother me if the Health District does not have record of it, except if I want to sell in the future. It may effect the value of the property if there is not an approved system.
Depending on how much was added to the home. One room, two or more? Added bedrooms again could be a challenge if the septic system was designed for 2 o3 bedrooms and now there are 5 bedrooms in the home. I would also encourage you to get a home inspection done and make sure the inspector is an ASHI member.

Steve R Halvorson