BELFAST — Athenahealth got city approval last month to convert the interior of warehouse next to the company’s main building into offices and a new “Operations Lodge” for mail processing. A nearby parking area would be expanded to add 45 spaces.

Taken alone, it’s an unremarkable project. Most of the construction will happen inside the garage-like interior of the building, which will be divided into two floors. The exterior will see only minor modifications. But the decision to convert the outbuilding on the former MBNA property hints at the arc of a company that has kept a low profile in Belfast.

The Watertown, Mass.-based health insurance billing service first came to Belfast in 2008, moving into a corner of a 130,000-square-foot former MBNA building. Athenahealth wasn’t an obvious fit. A similar building across the property line to the west had been snapped up by Bank of America. Athenahealth, by contrast, had just 12 employees in Belfast at the outset. But those numbers began to grow immediately and have risen steadily every year. Vacant “pods,” or wings of the building, were colonized one-by-one. MBNA’s telltale dark-green-and-beige-striped wallpaper was replaced with a solid palette of earthy including athenahealth’s spring greens and yellows.

All of this has happened quietly. Employment milestones were occasionally noted in the business sections of local newspapers. But aside from a 2011 purchase of Point Lookout Resort in Northport — another former MBNA property — the rate of the company’s growth here has probably been most evident to employees trying to find parking spaces outside the Belfast offices.

Like the gestation of the company’s namesake in her father’s skull, the titanic building has been gradually filling up. 

The Operations Lodge expansion is the first major development outside of the walls of a building that once seemed impossibly large. When renovations are complete, the facility is expected to house 150 workers.

For perspective, that’s about the total number of employees at Front Street Shipyard, a business that has been not-unfairly credited for lifting the economic tide in Belfast in recent years. 

”If you suddenly had a new employers bringing 100 jobs to town, that’s usually a very major announcement,” said City Planner Wayne Marshall.

Marshall recalled a recent conversation in which he’d been asked how many employees athenahealth had in Belfast. He guessed around 650.

“I was way shy,” he said. “It’s over 800.”

Bridger McGaw, director of athenaEnvironment at athenahealth, said the company crossed that milestone on December 18 and is likely to continue to expand its workforce in Belfast.

Athenahealth has offices in Watertown, Mass.; Atlanta, Ga.; San Francisco; Austin, Tx.; Princeton, NJ.; Birmingham, Ala.; Durham, N.C.; and Chennai, India. Belfast accounts for a little over a fifth of the total workforce of 3,702. That number excludes temporary employees and interns.

The facility here is unique in that it handles all of the company’s insurance claims and doctor’s bills. More than 1.5 million pieces of mail pass through the “Remittance Processing” department in Belfast each month. Those operations will ultimately move into the new building.

So, does that mean the massive former MBNA building is full?

“Not exactly,” said McGaw. “This specific business unit needs more space to grow, so we are repurposing an existing structure to free up space for the other teams to grow in the main building.”

Athenahealth’s growth in Belfast is showing up in other small ways. A plan to expand the municipal airport is almost entirely driven by the company, according to the Belfast’s Economic Development Director Thomas Kittredge.

“They were the catalyst for that,” he said. “They expressed interest in a longer runway.”

Kittredge noted that other groups use the airport, including clients of Front Street Shipyard, but it was the regular trips by athenahealth staff and a larger plane owned by the company that couldn’t land here drove a longterm recommendation to extend the runway from 4,000 feet to 4,700 feet.

“It speaks very well that they can find the employees they need,” he said.

Ethan Andrews can be reached at news@penbaypilot.com