FREMONT — A tax hike proposed by the Alameda County Mosquito Abatement District could wind up costing taxpayers more than $5 a year.

A ballot mailed to property owners in the district last week includes a clause that allows the district to increase its annual assessment by up to 3 percent a year to account for inflation.

The current assessment, which sets maximum tax rates for homeowners at $1.74 a year, does not allow for increases to keep up with inflation.

The proposal now before property owners would increase the maximum assessment the district can charge homeowners this year from $1.74 to $5. However, depending on inflation, homeowners could end up paying a little more.

For example, if next year the San Francisco Bay Area Consumer Price Index shows that inflation is 3 percent or higher, the district could increase the assessment by 3 percent, thereby charging single-family homeowners a maximum of $5.15 a year.

Also, the district can adjust for inflation retroactively in future years.

For example, if in 10 years the district for the first time increases the maximum assessment to more than $5 a year, it can calculate the inflation increases it could have collected during the previous decade and include them as part of the new tax. In such a case, if inflation is high, the district in 10 years could increase the maximum tax for single-family homeowners by more than 30 percent, to nearly $7 a year.

The district can start accounting for inflation next year based on the San Francisco Bay Area Consumer Price Index.

District Manager John Rusmisel said the district doesn't expect to use the inflation clause for the foreseeable future.

The current annual assessment has been in place for 25 years. During that time, the district has charged the maximum assessment only six times.

Rusmisel said the increased assessment is necessary to prevent staff cuts and to effectively combat the West Nile virus.