Related Beal's 370-380 Harrison Ave. project under construction in the South End will include 52 affordable apartments and three income-restricted condos among its 273 residential units.

Boston’s most powerful policy weapon to encourage private development of affordable housing is continuing to pay off, so much that officials are considering whether to change the requirements for the second time in three years.

Originally enacted in 2000, the inclusionary development policy requires multifamily developers to include a minimum 13 percent income-restricted units in new projects.

Boston Mayor Marty Walsh signed off on a revised policy in 2016 that raised fees on developers that don’t include on-site affordable units in the city’s highest-priced neighborhoods.

The changes don’t appear to be dampening development. There were 546 IDP units completed in 2018, which represents 21 percent of the total units created in program’s 18-year history, Boston Planning and Development Agency Brian Golden noted at a press briefing Monday.

Another 2,119 IDP units are under construction, have received building permits or been approved by the BPDA.

The bulk of the development has been in the central neighborhoods from the Fenway to the Seaport.

“Since the city of Boston has few available parcels in these neighborhoods, and land prices are out of reach of affordable housing developers, the IDP is a critical tool in creating income-restricted housing in downtown neighborhoods,” the BPDA’s 2018 annual report on the IDP states.

Given the strong development pipeline, BPDA officials have had informal discussions with developers about revising the policy, Golden said.

Under the existing policy, developers can include 13 percent affordable units on-site, pay a fee to support affordable housing projects in surrounding neighborhoods, or a combination of the two. The “cash-out” fee for projects that don’t include affordable units ranges from $200,000 to $380,000 per unit, depending upon the neighborhood.

Tim Davis, the BPDA’s housing policy director, said the cash-out option is only offered to developers who share their pro formas with the agency and demonstrate the project wouldn’t be economically viable with on-site units.

Developers contributed $13.5 million to the IDP fund in 2018, which represents nearly 10 percent of the total funds collected since 2000.