There are many systems that get you within +/-25% for 90% of the houses. Yours is probably in that range, depending on what time period you choose.
If you choose a whole year, your assessments will lag badly in a fast moving market. If you choose a shorter time period, you risk the small number of sales making the measure a noisy one. I suspect the whole year basis is a better balance.
Then of course, there's the "is a garage livable area? is it land? is it neither?" "is a basement livable area?" "what makes an attic livable area vs storage?" "what about porches? patios? sheds? gazebos?"
Taxes are usually assessed and paid once a year, so I would go with one year.
Livable area is already very well defined in law, so no need to do it again. Every real estate listing has this number. There's even an ANSI standard Z765-2003 for it.
If you choose a whole year, your assessments will lag badly in a fast moving market. If you choose a shorter time period, you risk the small number of sales making the measure a noisy one. I suspect the whole year basis is a better balance.
Then of course, there's the "is a garage livable area? is it land? is it neither?" "is a basement livable area?" "what makes an attic livable area vs storage?" "what about porches? patios? sheds? gazebos?"