.Learned after purchasing a townhouse that an addition was added, many years ago, without a permit. Can I be held responsible for this illegal procedure?

No, but if you want to sell the property you may need to get a Certificate of Occupancy.

You might want to pull a permit and have the addition properly inspected and passed by the county and correct anything in order to get it passed if it does fail inspection.

Just relooking at this, but I suspect you did not have an attorney represent you at the closing. An attorney would have have pulled the physical description of the property before closing and made sure you had COs for what you were purchasing.

If the seller misrepresented that the addition was approved, you might be able to sue them.

But, since you bought the property without checking its now your responsibility to to get the CO unless you sell to someone as unwitting as you were.

There is the possibility that, when inspected, the addition may be found illegal and you will have to demolish it.