Under Proposition 13, a new base year value is established for the portion of a property that undergoes a “change of ownership”, and a separate base year value is established for any “new construction” completed after a change of ownership. As of July 1, 1983, taxes are based on these new values starting on the first day of the following month. These taxes come in the form of a supplemental assessment, which is determined by taking the new value and subtracting the total value already assessed to previous owners for that time period. If the new value is higher than the total value already assessed, additional taxes will be due. However, if the new value is less than the total value already assessed, taxes will be refunded. This is referred to as a negative supplemental.