CAMBRIDGE — For folks who work in downtown Cambridge, Katie Mae’s luncheonette at 516 Poplar St. is a kind of institution. Opened in 2014, Katie Mae’s filled a quick-food, simple-food gap in the downtown Cambridge menu of restaurants, and the tiny storefront rapidly became popular. So popular, as a matter of fact, the only real complaint about the place was the time it took to get a seat or to get served. It wasn’t long before Katie “Mae” Smith, proprietor of the restaurant, knew she was going to have to expand in order to serve downtown properly. We started thinking about expanding last year,” Katie told us. “People were coming in for lunch and we didn’t have enough seats. So we started looking around for another place.” In order for the new Katie Mae’s to be viable, they needed a dining room with seating for about 100 people. A number of places were considered, but they took a long look at the Provident Bank building, on the corner of High Street and Poplar Street. While the Dorchester Chamber of Commerce takes up half of the downstairs footage, the old bank side would be easily big enough for their uses. Katie and her partner Chuck formed a corporation they call Park Place LLC, and purchased the building. So far, the plan is to retain the Chamber in their downstairs office, and to renovate the offices upstairs to make them more marketable to business clients. “There can be eight offices upstairs, or so,” Katie told us. “We’re installing an elevator, since it’s required by law for certain office tenants.” She admitted that office renovation was going to take a back seat to getting the restaurant set up and operating. In the meantime extensive renovations are taking place in the old bank building. They’ve removed all of the teller cages and are refinishing the floors, and building out what will be a brand new kitchen in the back. Katie doesn’t see a tremendous change in her menu in the future. “We’re going to stick to what we know,” she said. That includes homemade, Amish-inspired “comfort food” and sandwiches. “We plan to have special dinner nights, like steak night, for example. But mostly we’re going to stick with what we know everybody likes.” One thing that will change is the hours at Katie Mae’s. The new location is planning on keeping the doors open from 8 a.m. to 9 p.m. “We’re going to plan a lot of our specials for Monday and Tuesday nights,” Katie said, “since downtown is kind of slow on those nights. So we’re hoping we can get a crowd down here and bring downtown back to life on those two nights. “We also plan to have weekend nights where we have a jazz band here,” Katie said, standing in what will be the main dining room in the Provident Bank building. “We’re going to try to get the older crowd to come down into town; to dress up, put their hats on, and come down here and dance and enjoy some jazz music. We’re going to have church bands come and play. It’s going to be really nice. We have a lot of good things we’re going to try to do.” Work is progressing on the renovations in the building, and Ms. Smith says that they hope to have the doors open by June.