The Boston City Council on Wednesday unanimously approved a proposal from District 9 City Councilor Mark Ciommo, and championed by Mayor Martin J. Walsh, to increase the city's residential tax exemption -- a move that would lower the average property tax bill for a single-family, owner-occupied residence by $299 a year. The City Council approved the proposal 12-0 (City Councilor Tim McCarthy was absent and thus didn't vote). Homeowners will see the lowered rates reflected in their third-quarter tax bills, which will be sent out at the end of the year. The residential exemption for taxpayers who occupy their homes as their principal residence will increase from 30 percent to 35 percent of their homes' assessed value. The average property tax bill for residential taxpayers will decrease from $3,533 to $3,234, according to the city.