ST. PETERSBURG — Builders will break ground soon on a downtown apartment building the size of a city block, adding to a flood of new rental construction in an area developers are betting is a gold mine of young urbanites.Allen Morris Residential said Wednesday it would start construction in July on eight stories of apartments planned for 700 First Ave. S, where the Coral Gables development firm just bought 2 acres for $5.25 million in cash.Called the Hermitage, the $65 million project, expected to open by the end of 2015, would feature 348 one- and two-bedroom apartments, developers said. Rental prices were not disclosed.Named after an art museum in St. Petersburg, Russia, developers said the apartments would feature an "art gallery" in the lobby; a rooftop pool and spa; a fitness center; an outdoor grill; and a bicycle storage and service center. A coffeehouse would be built on the ground floor, with potentially other storefronts to come.Even in a downtown swimming in new construction, the Hermitage sticks out as the largest new apartment project started here since the housing bust. The Beacon 430, a four-story, 326-apartment complex at 430 Third Ave. S, is expected to begin leasing before summer. Modera Prime 235, an eight-story, 309-apartment complex at 235 Third Ave. N, scheduled its first move-ins for this month.Developer CEO W. Allen Morris said St. Petersburg's job growth and young energy made it "an undiscovered jewel," adding that he envisioned plenty of rental demand from empty-nesters, young professionals or "commuters who work across the bay in Tampa, but want the lifestyle of downtown St. Pete.""With all the apartments that have been built, all the apartments that have been proposed, there's still an undersupply," Morris said. "There were so many years of no development … that we are still way behind."The land, which once housed an old gas station, was bought in 2001 for $4.1 million by a small group of real estate professionals, seller Pete Apostolou said. It has since been used as an overflow parking lot.The development of the lot could continue downtown's westward expansion as retailers, barkeeps and restaurateurs outgrow the built-out and bustling Beach Drive area. The lot sits within a block or two of Tropicana Field and an old YWCA converted last year into Rococo Steak, an upscale restaurant.It is Allen Morris' first major residential project and first development in Tampa Bay, though it has developed dozens of offices, warehouses and retail centers across the state.Drew Harwell can be reached at [email protected] or (727) 893-8252.