Image courtesy of Elkus Manfredi Architects.

National Development has bought part of the site of a proposed 21-story tower in South Boston.

The Newton-based developer bought the 1-acre parcel at 323 Dorchester Ave. for $17.37 million with a $73.96 million mortgage from Santander Bank, according to public records.

National Development notified the Boston Planning & Development Agency in December of its intent to build 265 apartments, 47,000 square feet of office space and a grocery store and retail space at 323 and 345 Dorchester Ave. National Development plans seek approvals under a planned development area process, the company wrote at the time.

Taken together, the parcels span 3.5 acres. A National Development affiliate bought 345 Dorchester Ave. in 2018. The tower is part of a larger mixed-use project the company said it envisioned building in three phases.

The company owns several parcels in the immediate vicinity and last year declared its intent to redevelop 60-70 Old Colony Ave. into a Tatte Bakery and test kitchen and up to 10 other restaurants and retailers.