They told Jeff Tubbs that crossing Girard Avenue from Northern Liberties to build in Old Kensington was a crazy idea, especially in 2009, with the residential real estate market already deep in the recessionary tank.

Tubbs was undeterred, and the Flats at Girard Pointe at Third Street and Germantown Avenue was built. The prices there were groundbreaking as well: $300,000 to $450,000 in an under-$100,000 area.

Six years later, in fourth quarter 2015, the median sale price for the 19125 zip code encompassing that part of the city was $280,000 on sales of 129 homes, according to Berkshire Hathaway Home Services Fox & Roach Realtors' HomExpert Market Report.

To say Tubbs was more prescient than crazy is an understatement.

On a warm March morning, a walk of a dozen or so blocks north, east and west of the Girard Avenue El station quickly found scores of low-rise structures of steel and glass, today's materials of choice, in progress or complete.

Already spoken for by developers were factory buildings and vacant lots.

"A friend is turning that into rental apartments," Tubbs said, pointing to the former Eugene Chernin Co. at 1401 Germantown Ave. as he sat at a table inside Reanimator Coffee across Master Street from the factory building.

The friend, developer Dor Berkovitz of G-8 Life, said the Chernin building, also in Old Kensington, would be rehabbed into 50 residential units - studio, one-bedroom, and two-bedroom - with 11,000 square feet of retail space on the first floor.

No longer at the edge of neighborhood redevelopment, the 19125 zip code encompassing Fishtown and Kensington and, increasingly, adjacent 19122 form the center of it.

"Today, everyone knows that Fishtown has a thriving arts and restaurant scene," said Sean Killeen, a partner and director of development for AGA Developers L.L.C., which built and sold Trenton Stables' six townhouses in the 2200 block of Trenton Avenue.

"But five years ago, it wasn't as obvious," said Killeen, whose group has started work on South Square, a 19-unit residential and commercial project in South Kensington. "It was the vision of the neighborhood and the community's support of development that has made this one of the hottest zip codes in Philly."

This area northeast of Center City "is "Philadelphia's poster boy for revitalization of a formerly working-class neighborhood by educated and creative-class millennials," said Kevin Gillen, chief economist at Meyers Research and senior research fellow at Drexel University's Lindy Institute for Urban Innovation.

The lower cost of living in Fishtown and resurgent portions of Kensington "puts renting or owning there within reach," he said.

As Center City real estate has gotten too pricey for many younger buyers, the population of both zip codes has continued to grow, Gillen said. In 2010, 19125 had a population of about 22,900; today, it is estimated at 24,000 and will likely reach 25,000 in 2021. In 19122, a 2010 population of 21,500 is up to nearly 23,000 today and is forecast to close in on 24,000 by 2021.

Tubbs and his partners have four projects underway in Old Kensington and East Kensington.

"The goal was to have them staggered, but there were zoning delays," said Tubbs, who also started Urban Roots, a nonprofit that builds urban redevelopment projects and is creating playgrounds in South Philadelphia.

One such project, American Studios at 1300 N. Cadwallader St., is 90 percent complete, with three condo units sold, he said.

A townhouse with parking will be listed for $500,000, and a bilevel condo with a roof deck will be priced in the mid-to-high $300,000s, Tubbs said.

Yet to be started, he said, is 10-unit Hope Street, to be built with American Studios' partner Duling Construction Management. Prices will range from $250,000 to $295,000.

Tubbs is partnering with Domestic Goods, Artistry Residential, and E-Built L.L.C.'s Jim Maransky on the Frankfordian at 2006-2010 Frankford Ave., which has six two-bedroom, two-bath bilevel units priced in the low to mid-$300,000s, as well as an 1,800-square-foot bilevel commercial space.

They also are partnering on Frankford Stacks, a condo project in the low to mid-$300,000s at 2012-15 Frankford Ave. and 2012-15 Blair St.

Maransky, best known for the Icehouse, his 36-unit multi-phase condo project on East Thompson Street, had first sought contiguous parcels in Northern Liberties and came across the boarded-up Fishtown bar of the same name, plus adjacent properties. It took two years to assemble the 32,000-square-foot site, which is being completed after a decade.

Unlike Northern Liberties, Fishtown has "quieter streets, less traffic, and is a more 'neighborhood-ish' area very attractive to development," he said.

These neighborhoods have become the first choice for buyers priced out of Northern Liberties, where new construction under $500,000 doesn't exist, as well as "young couples who may be starting a family," Maransky said.

Those buyers, said Chris Somers of Re/Max Access, "like the fact they can get new construction with a 10-year tax abatement, perhaps parking, and a roof deck vs. an older home in other areas."

First- and second-choice buyers "have essentially doubled demand," Somers said. "The buyer pool remains high, and new-construction inventory is low since everything sells so quickly."

Kensington, too, is on fire, said Nino Cutrufello, development director for Callahan Ward Co., which also builds in Northern Liberties: "There is barely any land in the few blocks north of Girard that hasn't changed hands in the past few years."

Two years ago, Callahan Ward purchased a vacant lot near Fifth and Master Streets for $35,000. A year ago, it bought two lots near Fourth and Master for $50,000 each.

"Last week, we lost out on a bid to purchase two lots on Leithgow Street just north of Girard Avenue for $100,000 per lot," Cutrufello said - $20,000 more per lot than asking price.

Said Tubbs: "There is great energy here. The millions of dollars in investments, the public transit, and existing infrastructure make this an amazing place to be."