As South End emerges as one of Boston’s office development markets, a longtime landlord has hired an outside broker for the first time to market three availabilities in the SoWa submarket.

Boston-based GTI Properties is seeking tenants to fill nearly 16,000 square feet at 560 Harrison Ave. and a pair of 11,000 and 8,000-square-foot spaces at 580 Harrison Ave. Boston Realty Advisors is representing GTI, currently the largest office landlord in the South End, in the first time GTI has hired a third-party brokerage.

The 6-story converted mill buildings contain a combined 300,000 square feet of brick-and-beam office space.

BRA’s Peter Bean, Bonny Doorakian and Robert LeClair are handling the GTI assignment.

Existing tenants within GTI’s Harrison Avenue portfolio include Boston Sports Club, Shawmut Design and Construction, McCarthy Mambro Bertino, Listen Inc., Machado Silvetti Architects, Gaslight Brasserie du Coin, Greenlight Café and Cinquecento.

With office vacancies in Boston dipping below 10 percent, South End has begun to attract development of class A office and lab space.

Burlington-based Nordblom Co. is building a speculative 8-story, 230,000-square-foot office building on top of the 321 Harrison Ave. parking garage. Boston-based Abbey Group is marketing 1.6 million square feet of office and lab space in a planned development at the former Boston Flower Exchange site on Albany Street.