Boston is expanding a program that encourages creation of additional apartments in existing homes and provides low-cost loans of up to $30,000 for such projects.

The Boston Planning and Development Agency directors approved a zoning amendment Thursday to expand the program throughout the city, with the exception of the downtown and downtown waterfront districts.

The pilot enacted in 2017 in Jamaica Plain, Mattapan and East Boston generated 55 building permit applications, 12 of which have been approved, the BPDA said. The vast majority have been for basement conversions, but the program also allows attic conversions and carve-outs of new units, according to the BPDA.

The pilot emerged from the Boston 2030 master plan as a low-impact way to increase housing production and provide affordable living arrangements. The so-called “additional dwelling unit” (ADU) program allows one additional self-contained independent living space within a residential structure’s existing footprint. ADUs are required to include a bathroom, kitchen and two means of egress, and to be added to the city’s rental registry.

The Office of Housing Innovation has launched a webpage with details on permitting, cost estimates and eligibility requirements.

Zero-interest loans up to $30,000 per project are available through the Boston Home Center to households earning at or below 135 percent of the area median income, with no payments required until sale of the property or a cash-out refinance. As part of his fiscal 2020 budget recommendations, Mayor Marty Walsh asked that $650,000 be allocated for the loan program.

The Boston Zoning Commission has final approval and is expected to review the policy at its May 8 meeting.