When Jean Harrow got a ticket in 2016 for unauthorized renovations to her Queens home, she thought it was a misunderstanding. Yes, she had put a powder room in her basement without realizing she needed a permit. But surely, she said, she wasn’t responsible for the washer and dryer a previous owner had installed downstairs — illegally, according to the $1,600 citation. She would simply explain that at her hearing.

As she waited to do just that, Ms. Harrow got a second ticket — for “failure to comply” with the first. In the 14 months after the original citation, she received five others for the same issue: $15,600 in additional fines. Each meant another hearing, and although she never missed a court date, the tickets kept coming.

Thousands of small property owners in New York City have been hit with a similar pileup of fines, an unintended result of a decade-long crackdown set off by fatal construction accidents. In recent years, the city’s Buildings Department has hired hundreds of new inspectors and doled out harsher penalties for violators. But rules introduced as a safeguard have become a costly trap for ordinary people, The New York Times found.

A review of public and internal records, and interviews with current and former city employees, reveal that some of the toughest punishments have had less to do with property owners’ flouting safety rules than with their confusion over how to respond to that first ticket. Any delay or misstep can lead to a series of fines that will snowball until the owner certifies that the violation has been set right. But the path to certification takes time — and money.