All taxable property must be assessed at 100% of current market value less statutory exemptions (such as Homeowner, Agriculture or Timber exemptions). The Assessor must estimate the most probable selling price of properties as of January 1st of that year. To do this, the Assessor considers construction costs, the income from income producing properties and sales data. Some of the features that influence sales price are size, location, age, quality of construction and overall condition. See Idaho Code 63-205
The assessed value of your property is not the only factor in determining your property tax liability. Your assessment could stay exactly the same and your tax bill could still increase. This is because taxes are budget driven, not assessment driven. The assessed values are a means of distribution of the tax burden.
Structures do depreciate with age, however, the market appreciation usually overrides the physical depreciation.
The law does not limit the amount a property assessment can change. The taxing district’s budgets are limited to a 3% increase over the previous year. The taxing districts that are in your area are listed on your assessment notice with their phone number and budget hearing dates.
Things that can affect the value of your property are a remodel, an addition to the home or adding other structures such as a garage or storage building. The market is a primary driving factor. The Assessor’s office follows the market by studying sales and applying that information to determine the most probable selling price.