Whenever you buy a home or complete new construction — including any renovations or additions — state law mandates that the San Francisco Assessor’s Office reappraise your property. The difference between this new assessment and the prior value of the property is called a supplemental assessment which is an additional tax that is billed separately from your regular property tax.

However, supplemental taxes are prorated so that you only pay for those months of the fiscal year remaining after you have taken ownership or completed new construction. At the start of the new fiscal year, your supplemental assessment then simply becomes part of your regular — and now proportionately higher — annual tax bill. The only exception to this rule is an improvement in the seismic strength of your home: earthquake retrofits that raise the value of your property are exempt from regular reappraisal requirements.

Unfortunately, you should be aware that supplemental tax assessments could arrive at any time of the year. They are not on the same twice-yearly installment schedule as your regular property taxes. In addition, if you pay your taxes through your mortgage company, note that the City sends supplemental tax bills directly to homeowners. Your mortgage company will not get a copy, so you shouldn’t overlook this important piece of mail.

For more information, see the Web site for the Office of the Assessor-Recorder.