Home security company SimpliSafe has signed a lease expansion for 20,271 square feet at 294 Washington St. in Boston’s Downtown Crossing, effectively doubling its headquarters to accommodate growth after moving from Cambridge in late 2014.

SimpliSafe looked at various locations in and outside Boston before deciding to expand at its existing headquarters, said Ben Sutton, a vice president at NAI Hunneman.

“Being literally on top of the (MBTA) State Street station was a huge boost to their hiring. It was convenient and they wanted to keep it that way,” Sutton said.

In 2014, Boston-based Synergy Investments and DivcoWest acquired the 227,233 square-foot class B office building built in 1903 and installed a new lobby, gymnasium and conference room space.

Proximity to public transit and scarce vacancies in East Cambridge have increased demand for class B office buildings in the Financial District and Downtown Crossing neighborhoods. The availability rate for class B and C office space in East Cambridge was 8 percent at the end of 2015, according to Cushman & Wakefield research, with average rents of $55. In Boston’s Midtown submarket, average class B rents were $46 with an availability rate of 11.6 percent.

“Pricing is secondary to just being able to find convenient locations in a growing cluster of tech firms,” Sutton said.