Another Park Slope resident, Rose-Marie Whitelaw, turned her entire 10-by-20-foot deck into a haven for her seven cats. Using pipes, chicken wire and deer fencing, she erected a seven-foot railing that the cats cannot climb, then spray-painted it black so it would be less obtrusive.

“I’m kind of handy with copper piping and a blowtorch,” said Ms. Whitelaw, 50.

She and her husband, Russell Piekarski, have an outdoor picnic table where they eat meals among the cats in nice weather. Sliding glass doors lead to a kitchen and home office, and the cats can usually go in and out all year round.

“When the snow is piled to here, Julius makes tunnels,” said Ms. Whitelaw, referring to her large orange tomcat.