House purchase with unpermited addition

[Peter Sieracki]

Hello,

I am going through due diligence on a residential property and have a situation. The house was built in 1955 and has a substantial addition. The city records department does not show any permits being opened or closed in regard to the addition. The county assessor shows the addition as part of the property being EFF the only time the house was sold in 1974. The seller states that the addition was on the house when she bought it.  
How can I investigate further?

Thanks, in advance for any perspective you can share