re 25 Athena Court, Mahopac, NY 10541 you do not give a square footage $ based on local comps

Margie19 - you have a common misunderstanding regarding Zillow. Zillow displays information for most properties in the United States. They collect what information they can find and that is what they display. Often the most recent information is from the last MLS listing. This property sold last year (I am Realtor who handles the Mahopac area) and that is the info reflected for the property. If it has been since improved and you have an interest in the property (like, say, you are the new owner), you can 'claim'  the property and update the information. This may affect the value, but the Zestimates are mathematical guesses based on complicated algorithms that incorporate all current market data, and, sadly, the market is going down, not up.

Just keep in mind that Zillow gathers and analyzes reported information across the country, they know nothing else about the property. If you have questions about current market value for this property, you may contact me directly - it's my area of expertise.