I've done several dozen successfully with my best assessment reduction over $300K. Several methods work:
1. Best - Pay for an appraisal from a PA licensed appraiser. He will provide recent comps adjusted to come up with an appropriate value of your house. PA license they really can't argue with the value he sets on the property.
2. Brokers opinion report. Real estate broker pulls comps and adjusts the values to come up with a value for your house. Doesn't charge as much as an appraiser, doesn't carry as much weight. Can also use a real estate agent but has even less weight than broker.
3. Purchase price of the house if it was bought within the past three years and you didn't pull a building permit for major renovation.
4. Comparable sales pulled by yourself. Sales should be within past three years and within one mile of the house. Can use Zillow or other internet search to find the sales. Hopefully your county has assessment information on line to be able to compare the property attributes, Zillow has some info but not as much as the county. You can go to the county building to get property cards owned by you or not. Comparison includes:
a. Square footage - by far the highest value attribute. Square feet of living area is worth significantly more than basement or attic.
b. Grade of exterior can swing value significantly
1. Stone or other fancy finish
2. Brick or hardiplank
3. Vinyl siding or wood
4. Wood - old style painted
c. Condition - another that can vary the value, definitely very subjective. Most people focus on pointing out all the little things wrong but I've never felt that swung the assessor more than a little bit. They look at
d. lot size
e. garage integral or detached, other out buildings never swings more than $10K to $15K
f. porch or deck space by square feet
g. Number of baths or sometimes plumbing fixtures, only worth a few thousand
h. fireplace only $1K to $1.5K
* Pull your property card and look for mistakes to argue.
5. Compare your assessed value to your neighbors, not a high success rate but sometimes works --96.236.xxx.xxx