According to a 1990 US Census report, 20 percent of New York homes (about 1.5 million) have septic systems (though I suppose not the ones on the NYC plumbing grid), which is on par with the national average, according to Cornell University.
Prior to employing a plumber or septic system specialist, be sure you are aware of local regulations. The Cornell website offers a wide range of information on building and maintaining septic systems as well as New York regulations. For example, if you are buying or selling a home with a septic system an evaluation of the system must be performed. Such information as age, type (concrete, plastic, or steel), and an original design of the home’s water and septic system needs to be gathered by a professional and provided to the buyer. Other information you need to provide includes “soil conditions, drainage, seasonal high water table, and flooding possibilities,” states the website.