New York City's luxury real estate market cooled considerably this year, with the number of high-end sales down 18 percent, realtors Olshan Realty Inc said Tuesday.

Olshan blamed co-ops for the decline, as demand has shifted to condos instead. But even still, the "golden years" of new condo construction have passed, the firm added.

A total of 1,102 properties had contracts signed for $4 million or more this year, down sharply from 2013-2015 but still higher than 2012, before the condo boom started.

Average days on the market rose 30 percent, meaning it took an extra two months to sell a luxury apartment this year versus 2015. The average asking price also rose, but the average decline from asking to contract actually increased a bit.

Still, though, there's enough demand to sell apartment sight unseen. Olshan said 58 percent of all condo sales were new construction sold off just a floorplan.