This may be a case of the county tax assessor not having kept up with improvements on your property. Believe me they'll catch up eventually and it may in fact have an effect on your taxes. The fact that you have had an appraisal would not have an effect on this process, believe it or not. They look at the most recent sale of the subject property (yours) and sales of nearby comparable properties.
The assessor is required to inspect all properties in the county, and they may behind in your area.
I'd be happy to look at your specific situation if you want.
Jim Hughes Real Estate
First, what I hear you saying is that Pottawattamie County's assessed valuation of your home is below the market value you have established from an appraisal. This is actually quite common, as these two indications of value are not directly related to each other. An appraiser's opinion of market value estimates what it might sell for. The assessed valuation is determined by the county assessor, and is the value you are taxed on. If you have made improvements (and you took out any required permits!) you will likely immediately improve the market value of the home, but this might not get reflected in the assessed value immediately.
You also seem to be indicating that you have contacted real estate agents who, for whatever reason, have not been interested in listing your home. I, for one, would be delighted to visit your home and then prepare a CMA (comparative market analysis) of your home, including recent sales of similar properties and active listings of similar properties currently on the market. There are any number of qualified and ethical agents in this area that could help you.
Let me know if I can be of assistance.
CBSHome Real Estate