In Manhattan, the drops were stunning. July contracts for co-ops priced between $4 million and $5 million were down 94 percent from the same month last year. No contracts were signed for condos above $20 million. Across all price brackets, condo and co-op contracts were down 56 percent.

But in Brooklyn — where New Yorkers often migrate to in search of bigger homes and more open space — the numbers surged: 178 new condo contracts in July, up from 147 last ...