How many visitors to coastal Marin spend the night? The conversation around November’s ballot measure that proposes to increase the transient occupancy tax in West Marin has oft cited that just 3 percent of tourists are overnighters. Yet in the absence of available data from the National Park Service or the county, that number is mere guesswork.

The claim that 97 percent of visitors do not stay in short-term rentals has supported the argument by the West Marin Chamber of Commerce—which has vehemently objected to the proposed tax increase—that a hit to hoteliers and short-term rental operators is unfair. The group has said a broader move, such as a sales tax hike, would more fairly distribute the burden. 

Jeff Harriman, owner of the Tomales Bay Resort and Marina in Inverness and a longtime member of the West Marin Chamber of Commerce, said the chamber first made the calculation back in 2004—the last time the county increased the T.O.T., from 8 percent to 10 percent. 

Mr. Harriman explained the calculation this month, though he also cautioned that it is outdated. In 2004, he determined that there were 350 rooms in West Marin, a number he said was based on how many people paid the T.O.T. at that time. Assuming a 50 percent average occupancy and an average of 2.2 people per room—numbers he said are true for his resort—he calculated that 141,000 people spend the night in West Marin each year. 

Comparing that figure with the park’s estimate of 2.48 million visitors to the seashore last year and an additional ballpark 1.24 million visitors to other villages in West Marin, Mr. Harriman concludes that 3.8 percent of total visitors spend the night in West Marin. 

Roy Given, the county’s tax collector, said the county doesn’t have the data to make such a calculation. 

Mr. Given has worked this year with the Silicon Valley company Host Compliance to gather more information on the county’s short-term rentals in order to enforce compliance with the T.O.T. The effort has focused on collecting names and addresses of everyone who operates a short-term rental—intel that sites like Airbnb, VRBO and Homeaway do not fully disclose and that the county needs in order to enforce taxation independently of them. 

But even though the county doesn’t have all of the information, operators are not necessarily out of compliance. The county receives the T.O.T. for the users of Airbnb, which has endured many lawsuits nationwide and willingly signed a contract with the county nearly two years ago, though it receives one check without individual details. Other platforms like VRBO and Homeaway have been unwilling to sign  such an agreement with Marin, leaving registration up to individuals.

Mr. Given plans to roll out a new compliance program in November, targeting  operators who have not been paying the T.O.T. 

How big of a wave will that make? According to the finance department, there are currently 502 short-term operators in District Four, which includes some eastern areas of Marin. There are between 300 and 400 short-term operators throughout the entire county that are not registered with the county, Mr. Given estimates. 

It is worth noting that the latter estimate only accounts for operators who have not registered for a T.O.T. certificate with the county; there may be others who are registered with the county, or through Airbnb, but are underreporting the number of days they rent—a loophole Mr. Given’s new program will address. 

Though Host Compliance’s research effort may eventually yield the exact number of rented rooms in West Marin, Mr. Given did not offer a guess at this time. 

Many other aspects of the chamber’s calculation, such as average occupancy rate or people per room, would require a fair amount of further data beyond what Host Compliance plans to obtain.

Point Reyes National Seashore spokesman John Dell’Osso said the only statistic about overnight versus daytime visitors available to his knowledge is a Sonoma State University survey completed over an 18-month period from 1996 to 1998. 

That study showed that between those years, an average of 22.5 percent of visitors to the seashore spent the night. The numbers varied by season: of 488 people surveyed in the summer, 29 percent were overnighters; of 68 people surveyed in the winter, 13.2 percent stayed the night.

The absence of solid numbers is a pressing problem for the Chamber of Commerce. “We have never had good information for how we would determine this,” Mr. Harriman admitted. “Unfortunately, we won’t have this information before voters have to decide on Measure W.”

The measure would raise the transient occupancy tax that short-term rental owners pay the county, strictly in West Marin, from the current 10 percent to 14 percent. Monies generated by the increase will be earmarked for emergency services and affordable housing, distributed by the county in conjunction with a local advisory group. The measure requires a two-thirds majority to pass. 

The Marin Convention and Visitors Bureau has commissioned the Marin Economic Forum to complete a new study on the impacts of tourism in the county, including answering the question of overnight versus daytime visitors. Representatives from the bureau declined to provide details, stating that the project was in its early stages, but Mr. Harriman, an original board member for the bureau, said the study would cost $15,000 and likely be completed by early next year. 

An earlier report by the Marin Economic Forum, written at the request of Supervisor Dennis Rodoni, who first proposed the tax increase early this year, addressed the projected impact on local businesses. 

Released in July, that report stated that the hike was “unlikely to reduce hotel demand significantly.” It did acknowledge some possible impacts to hoteliers and made a few recommendations to mitigate them, including increasing marketing.  

But the report did not shed light on the exact number of overnight versus daytime visitors, a fact that Supervisor Rodoni said this week “would be good information to have.”

The supervisor, who has taken a lot of heat from the hotel and bed and breakfast industry since he first unveiled the proposal, emphasized that he has tried to ease concerns by newly including local campgrounds in the tax increase and by supporting the county’s larger efforts to bring all short-term operators into compliance. 

“Though the exact number of visitors that are staying is hard to verify, I realize that the way to tax all of them would be to raise the sales tax—but that taxes residents too,” he said. Not only was he not prepared to do that, he said, but any reasonable increase to the sales tax would not generate the same level of funds as the T.O.T. hike, which will pull in an estimated $1.3 million a year.