In the spring, Philadelphia's single-family housing market had its best quarter in a decade, with prices and sales volume surging throughout the city.

The average house value "soared" by 7.3 percent in 2015's second quarter compared with the first three months of the year, said Kevin Gillen, chief economist of Meyers Research and senior research fellow at Drexel University's Lindy Institute for Urban Innovation.

Based on single-family home sales data between April 1 and June 30 from the city Recorder of Deeds, Gillen said, the quarterly price rise was the largest since second quarter 2005, at the height of the real estate boom.

"With this increase, average Philadelphia house prices are up 5.4 percent from where they were a year ago, and 14.4 percent since their bottom in the first quarter of 2012," said Gillen, who tracks the real estate market throughout the Philadelphia area. (His analysis of second-quarter sales in the region will be available in two to three weeks.)

Spring 2015's median price of $138,600 was up 18 percent from $117,500 in the previous quarter, and 20 percent higher than the $115,000 median of second-quarter 2014, Gillen said. (The report did not include sales of condos, which he defines as multifamily housing.)

Every city neighborhood saw an increase in price - the first time that has occurred since second quarter 2013, he noted.

"It is the hottest market I've seen in more than 10 years," said Carol McCann, an agent with Re/Max Millennium in Fox Chase.

Values, for the most part, remain stable, but they continue to climb in such Northeast Philadelphia neighborhoods as Fox Chase, Pine Valley and Somerton, McCann said.

"These areas are slowly creeping up in price due to limited inventory and an influx of buyers hopping off the fence and taking advantage of still-low interest rates and first-time-buyer programs," she said. Those programs, which began in April, help someone buying a $120,000 home "with just $1,667 out of pocket, with an interest rate of around 4 percent."

Mark Wade, of Berkshire Hathaway Home Services Fox & Roach Realtors in Center City, called the city sales boom "the most seismic shift I have witnessed in my 26-year career."

"It is taking place today, and will continue on through tomorrow and the next day," Wade said. "The shockingly low levels of inventory is just something I haven't seen before."

Said Karrie Gavin, of Elfant Wissahickon, who sells all over the city: "It has been absolutely insane."

In June, Gavin said, she closed on 17 houses sold in April and May. There have been multiple offers because of a combination of scarce inventory and abundant buyers, both first-timer and move-up buyers, she added.

In neighborhoods such as Northern Liberties and Fishtown, houses are selling faster than developers can build them.

Nino Cutrufello and Marcus Toconita, working as Callahan Ward in these and other emerging neighborhoods, are building in the 900 block of North Leithgow Street in Northern Liberties - townhouses selling for $600,000 to $700,000. Of their seven units, five have been built. Three have gone to settlement already, and one is due for closing.

Two sold for cash, with agreements signed before zoning approvals were received, Cutrufello said.

"I'm surprised at the speed of the sales," said Toconita, "without a model and all for asking price."

Gavin said she had the same situation in East Falls.

Chris Somers, owner of Re/Max Access in Northern Liberties, said the second quarter was his busiest in a decade.

"The new-construction projects we represented were selling out left and right, with multiple offers as buyers were out in full force," Somers said.

Sales volume grew in the quarter to 4,198 transactions, "up from 3,061 transactions in the previous quarter and well above the 3,466 in the same quarter a year ago," Gillen said.

This is the third time in the last six years that single-family home sales have climbed above the city's historic quarterly average of 3,811, he said.

Gillen cautioned, however, that the news about the second quarter "should be tempered with the fact that Philadelphia's housing market tends to proceed in fits and starts."

"While both recent numbers and current fundamentals give more reason for optimism than pessimism, it should not be surprising if further good news still experiences the occasional setback," he said.