Miami city commissioners on Thursday gave final approval to a substantial hike in so-called ''impact'' fees charged to developers with plans to use the money to improve the city's parks system.

Although charged to developers, impact fees are typically passed on to homebuyers. The fees are used by local governments to offset the costs of building new schools, roads and parks required with population growth. Most of the money collected by Miami's new fees would go toward parks improvements, including the purchase of new parkland. Miami is putting together its first parks master plan which, along with the fees, include Miami 21, the city's effort to create a ''smart growth'' development blueprint.

Miami had not updated its fees since the late 1980s. TischlerBise, a consulting firm that helped draft the new fee structure, projects annual fee collections will increase from $2.5 million to $7.3 million. The amount for parks would grow from $500,000 to more than $5 million a year, it said.

Developers argued that initial plans to make the new fees effective Jan. 15 placed an unfair burden on builders who already had budgeted projects with the old fees in mind. The city on Thursday agreed to allow a developer to submit only a building application by Jan. 15 to qualify for the old fees.

Miami's parks long have been criticized as inadequate. The Trust for Public Land, a national group advocating for open space, recently ranked Miami last among 12 high-density U.S. cities in terms of park acreage per 1,000 residents.

Nina West, chairwoman of the parks committee of Miami Neighborhoods United, a coalition of neighborhood groups, applauded the higher fee