Marin County is getting serious about making sure that property owners in the unincorporated area who rent short-term accommodations collect the transient occupancy tax.

The county hired Host Compliance, based in Seattle, to identify non-compliant short-term rental operators, and the company reported back with approximately 500 potential scofflaws. The data supplied to the county included the address of listings, the amount of revenue being generated from rentals and the owner of the property.

“The way the county’s ordinance is written it is the person who owns the property who is responsible for the taxes,” said Ulrik Binzer, the founder and CEO of Host Compliance.

Binzer, who until a few months ago lived in Tiburon, started Host Compliance in 2015. He said it now works with 150 cities and counties across the United States, including 50 in California, such as Mill Valley, San Rafael, Petaluma and Napa.

Binzer said people offering short-term rentals are constantly adopting new strategies to try to avoid paying government fees, such as not putting information on websites that identify addresses and moving from the most commonly known short-term sites such as Airbnb, VRBO and HomeAway to niche websites.

“It’s a cat-and-mouse game,” Binzer said.

He said that as a result, Host Compliance now monitors the nation’s 54 largest vacation rental sites.

On Wednesday, Marin County’s finance department mailed letters to the suspected offenders notifying them that they have 30 days to remit any tax money owed and register their property with the county. If they fail to comply within 30 days, they will be given an additional 10-day grace period before a tax lien is filed against their property to collect the money owed.

“I’m giving individuals an opportunity to come forward,” said county Finance Director Roy Given. “I’m not assuming they’re guilty until we have a conversation.”

Given said the county collects nearly $5 million a year in transient occupancy tax (TOT) revenue from about 530 people. Airbnb pays about $1 million of that to cover people listing properties on its site. Given estimates that when the county begins collecting from those who aren’t paying, yearly revenue could climb to $8 million or more.

The county has been trying for some time to get an accurate picture of how many short-term rentals are being offered in the unincorporated area and to get operators to register and collect the TOT tax.

In 2013, the county offered amnesty to people who were not collecting the tax. It agreed not to seek past unpaid tax if the landlords acquired a business license and began remitting the tax on a regular basis.

At that time, 212 operators were remitting the tax while another 217 were suspected of ignoring the law. Given says ultimately 69 operators took advantage of the amnesty.

Given said in 2013 under the TOT law the only way he could collect from people who weren’t paying was to take them to small claims court. He said at that time — before the county hired Host Compliance — he lacked sufficient evidence to win in court.

In August, at Given’s request, the Board of Supervisors adopted an ordinance empowering him to record a tax lien against the owner of any short-term rental property who neglects to collect the county’s transient occupancy tax.

For most of the county, the TOT is 10 percent, but on Nov. 6 voters approved increasing the TOT to 14 percent for properties in West Marin. Half of the revenue from the tax increase, an estimated $1.3 million annually, will be used for enhanced fire and emergency services in West Marin; the other half will be used to support long-term community housing in West Marin.

Advocates for the tax hike said West Marin communities are suffering greatly due to a critical lack of affordable housing, and short-term rentals have made the problem much worse. The tax increase was opposed by some innkeepers who say they are already experiencing problems competing with the proliferation of online, short-term rentals.

Given said that landlords who ignored the 2013 amnesty offer will be held accountable for three years of back TOT and may be assessed penalties and interest retroactively for that period. Interest is equal to one-half percent per month. Penalties amount to 10 percent per month and increase by an additional 10 percent each month.

Given invited more than 800 short-term rental operators to a workshop on Nov. 14 to brief them on the new policy. He said about 30 people attended the workshop. He said about 75 percent of the attendees indicated they were already remitting the TOT as required and the remainder indicated they weren’t.

“I didn’t go because I felt like I understand everything,” said Ken Drisdell, who rents a room in his San Rafael home via Airbnb.

“Every time I’ve called, the people in the tax department have been available, super friendly and easy with the rules to get you started,” Drisdell said. “Kudos to the county staff.”

He said using tax liens to collect from people who aren’t complying with the law seems reasonable as long as people are given adequate notice and an appeals process is in place.

“I’m into reasonable and being able to survive in this county,” said 70-year-old Drisdell, “because I need to be able to do Airbnb to be able to stay in my house and retire in place.”

Carol Zimmerman, who rents a room in her Mill Valley home via Airbnb, said, “Airbnb makes it really simple for us, since they collect it and submit it.”

“Renting one room in our house pays half of our mortgage,” Zimmerman said, “and allows us to stay in Marin.”