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A Boston developer submitted plans for a 70-unit office-to-apartments conversion in the Financial District.

The $21.5 million project is proposed at 60 Congress St. and 55 Kilby St., and would convert 49,105 square feet of offices on upper floors of the historic buildings.

Market-rate units would rent for approximately $3,700, according to a submission to the Boston Planning Department. The unit mix would be 50 studios and 20 one-bedroom apartments.

The development team is led by Patrick Mahoney, and previously worked on the development teams for projects including a 45-unit apartment building in Jamaica Plain and adaptive reuse projects.

The estimated project costs include $11.2 million for acquisition of the properties and $10.3 million for construction, and the application estimates it would face a 5.5 percent interest rate on a construction loan. A deed recorded at the Suffolk registry shows Mahoney’s LLC bought the properties in December.

The office space is primarily vacant, and all of the current office leases are expiring in the next 18 months, according to the application. A ground-floor cafe and commercial space would remain, the application states.

Boston’s downtown residential conversion incentive program offers developers tax breaks of 75 percent for 29 years provided developments hit the city’s 20 percent affordable housing set-asides.