Martin County’s taxable property value dropped $2.1 billion, about 9.5 percent, thanks to falling housing values. County property appraiser Laurel Kelly said the county tax roll is about $20.5 billion, about $1 billion short of what she estimated in June, and warned officials in Stuart, the county seat, that the drop is likely to continue as her office determines what property owners pay on their tax bills. A recent change in the state constitution has doubled the amount of value homeowners could avoid paying taxes on as part of homestead exemptions. The amendment also allows businesses to exempt up to $25,000 worth of personal property, such as office equipment, from taxation. That’s a major reason Stuart’s taxable value dropped 8.76 percent, the most of any incorporated area in the county, Kelly said.