A developer has filed plans for a six-story, 59-unit apartment building at 101-109 Boston St., right next door to the Fortress self-storage building with the giant inflatable lock off the Expressway near South Bay Mall.

The proposal by Cobalt Development Partners of Beacon Hill would actually use land now owned by Fortress - which would continue to maintain its large facility. In addition to apartments, Cobalt is also proposing 10 parking spaces in a lot, space for 59 resident bicycles and 12 for visitors and ground-floor restaurant/commercial space, according to its filing with the Boston Planning Department.

The units will range from studios to two-bedroom apartments, of which eight will be rented as affordable, or about 13.5% of the total. By filing this week, the developers grandfather the project from a new city requirement, which goes into effect Oct. 1., that would require 17% of the units - or 10 in total - be rented at reduced rates.

101 Boston St. filings and meeting/comment schedule.