Periodic reassessments are required by law. The Constitution of Virginia, Article X, Section 2 states, "All assessments of real estate and tangible personal property shall be at their fair market value, to be ascertained as prescribed by law." King William County is required to reassess all real property at least every 4 years per Section 70-45 of the King William County Code (Ordinance 02-21 adopted March 8, 2021).
The purpose of property tax reassessment is to insure every property is valued fairly and to realign those values with values of other kinds of property which are assessed annually, such as personal property, utilities, and mobile homes. The Code of Virginia requires that all property be assessed at market value. Because market value changes over time, periodic reassessment is required.
For detailed information, see the Code of Virginia, Chapter 32 - Real Property Tax, specifically Articles 5-7.