Miami 21, the city’s new zoning code, could finally go into effect this year, nearly four years after it was first unveiled. The code was approved last week. The plan asks developers to pay fees for additional height and density at their buildings, and it has been criticized by architects, who said it would lower design standards. Miami 21 will go before the Miami City Commission, which will make a final decision on the plan, within the next few weeks. City officials said they are ready to implement the plan if it is approved.