It turns out that the average property tax bill required to support BART’s proposed $3.5 billion bond measure on the November ballot could be as much as four times what the transit agency claimed.

Two months ago, I reported that the taxes needed to pay off that bond borrowing would be about double BART’s public estimate. Well, it could be even twice that.

That’s because legal language in Measure RR allows BART to issue bonds at up to the state limit of 12 percent interest, far higher than assumed for the previous estimates.

BART officials say they included that high-interest option on advice of their bond attorney because they wanted flexibility if market conditions greatly deteriorate. The district’s spokeswoman says BART never analyzed the effect on taxpayers of bond rates higher than 5 percent.

I estimate that if they were to exercise that worst-case option, homeowners could face tax bills averaging as high as $150 to $200 annually for nearly five decades.

Meanwhile, BART quietly stopped using its original erroneous tax bill estimates but never recanted them. Indeed, a statement issued by the transit agency attacking my July calculations revealed key staff members don’t understand the basis of their own estimates.

Measure RR is supposed to raise money for capital projects. BART claimed that to repay the bonds the average annual bill for an average home would be $37 in Contra Costa, $39 in Alameda County and $52 in San Francisco.

Actually, as I previously reported, using BART’s assumptions about bond interest rates and property assessment inflation, the tax bills would be about double that.

Under BART’s stated financing plans, the tax would last 48 years. That’s because the district plans to divide the $3.5 billion of bonds into 10 offerings spread over 18 years. Each bond offering would be repaid over 30 years.

To estimate how much property owners would pay, BART’s financial staff and consultants went through a two-step process:

First, they calculated a tax rate for each year from 2018 to 2065. To do that, they assumed the likely interest rate on the bonds would be 5 percent and the annual increase in property assessments would be 4 percent.

The result was an average tax rate of $8.98 per $100,000 of a property’s assessed value, and a peak of $17.49 per $100,000 in 2036. I did not dispute those numbers.

Second, transit district officials, to their credit, tried to translate those tax rates into more understandable average tax bills for homeowners. That’s where BART’s calculations went awry.

They took their estimated tax rates and applied them to an average house in each of the three counties to determine average tax bills for each.

For the calculation, covering 48 years, they used 2015 assessed values for each of the three houses but never increased them for inflation. Consequently, BART’s numbers were about half what they should have been.

In a statement defending their numbers, issued by spokesman Taylor Huckaby and Kerry Hamill, assistant general manager for external affairs, BART mixed up the two steps of the calculation.

The statement never addressed the second part of the calculation, which was the source of the error. Instead, they claimed I had accused them of failing to include an inflation factor for the first part of the calculation, the tax rates. In fact, I never questioned their tax rate calculation.

Perhaps I should have. Because now it turns out that Measure RR would allow BART to issue bonds at up to 12 percent interest. That would roughly double the district’s tax-rate projection. And quadruple its original estimates of the average tax bills.

Measure RR would also permit the district to stretch out the bond issues for as long as 40 years, rather than the 30 years used in the district’s projections. The longer payoff would increase the total interest costs to taxpayers by about 40 percent, but slightly reduce annual tax rates.

Doing both — increasing the bond interest rate to 12 percent and extending payments out to 40 years — would roughly double the annual tax rates and quadruple the interest costs.

Voters deserve to know that property owners could be on the hook for far more than BART claims.