That is one finding in the Center City District's annual "State of Center City Report," made public yesterday.

"We are not Miami, and we are not Las Vegas. This is not speculative housing; this is not investor housing," said Paul R. Levy, the district's president. "We know in general that 63 percent of Center City residents work in Center City and the prime attraction of people buying condos downtown is because they work downtown."

Throughout this decade's Center City real estate boom, Philadelphia's population of veteran cynics have asked two questions: Who is buying these things? What happens when the bubble bursts?

The answer to the first question, said Levy, is that Center City is being repopulated with young, affluent professionals and empty-nester retirees. The district's questionnaire to new condo purchasers showed that almost a quarter were retired.

As for the real estate bubble that popped in other U.S. cities, Levy said Center City appeared to have avoided that crisis, although he acknowledged that houses now took longer to sell than they did a few years ago.

"Housing prices have remained strong," Levy said. "There's no doubt there's a cooling in the market, but there's no evidence of a bubble bursting downtown."

Levy noted that Philadelphia has the lowest mortgage foreclosure rate - 0.49 percent - among the 10 largest metropolitan areas, far below Atlanta's 2.53 percent or Detroit's 4.92 percent.

Levy said the survey showed that last year the average Center City condominium sold for $428,596, while the average sales price for a single-family home was $286,616. Center City condominiums increased in price about 6.4 percent over 2006, while single-family homes increased in price by 9 to 15 percent, depending on neighborhood.

The number of Center City condominium units that sold for more than $1 million went from 49 in 2006 to 115 last year.

On the commercial side, Levy said, there have been positive trends, though the city still lags behind the suburban counties.

For the first time in 15 years, Levy said, Philadelphia increased its share of the regional office market - by 1 percent - thanks to the opening of the Comcast Center office tower.

Office vacancy rates continued to drop to 10.6 percent and the average rental rate for Class A office space reached $29.47 a square foot, a 14 percent increase over 2006.

Still, Levy noted, the increasing office occupancy rate was caused in part by the conversion of some Class A space, such as the conversion of the upper floors of Two Liberty Place into luxury condominiums.

Levy said Center City now had slightly more office space than it did 18 years ago.

Nor has Center City recovered as quickly as the suburbs from the national recession that followed the Sept. 11, 2001, terrorist attacks.

"We are still below 2000 levels in terms of office employment, so there's a rebound, but nowhere near as dynamic as the suburbs and not enough yet to offset the loss experienced starting from the recession in 2000-2001," Levy said.

But, Levy added, continued decreases in the city's business taxes - usually blamed for the loss of jobs to the suburbs - and the increasing price of gasoline could position Center City for another growth spurt.

"This is why I'm saying we're not gloomy about what is obviously a national downturn," Levy said. "It's not to say we're immune to it, but we happen to think we're incredibly well-buffered from it."