With West Contra Costa residents already paying some of the highest tax rates in the state for school bonds, and with aggressive projections about rising property values failing to materialize, school trustees should slow their massive construction program.

Current debt will likely force the district by 2014 to exceed the tax rate limit promised voters in 2002 and 2005 when they approved more construction bonds.

Rather than pausing, the school board last week voted to issue more bonds that property owners must repay. While legal, it's fiscally unsound.

Unfortunately, the district's school construction program has been plagued with deception. Five times in the past 13 years, officials obtained voter approval to issue bonds. Each time, the ballot analysis mentioned only the immediate proposal, hiding the ongoing costs of the previously approved measures.

All told, the district obtained voter approval to borrow $1.27 billion, of which it has issued $812 billion in bonds so far. Property owners must pay them back with interest. Under current forecasts, repayment of the full amount by 2050 will cost property owners close to $3 billion.

That might have been fine when times were good. But the school board's decision last week to soon issue another $100 million of bonds is the key reason property owners will pay 24 percent more this fiscal year for school construction.

The average homeowner's payment for school bonds will jump from $402 to $500. That's on top of the baseline property tax. And the bond costs will increase in coming years.

It's not just the absolute amount that's at issue, it's also the tax rate. When school districts seek voter approval for bonds, they estimate how much the borrowing will increase property owners' tax rates.

Usually, that estimate can be no more than $60 per $100,000 of assessed valuation. Thus an owner of a home assessed at $300,000 would pay no more than $180 annually for that bond issue. But school districts typically project that property values will rise and that resales will spark reassessments at higher levels.

If the actual increase in assessments falls below expectations, each property owner must be charged a higher rate to make up the shortfall. That's why property tax rates in West Contra Costa will soon exceed what voters had been promised in 2002 and 2005.

Even if the assessed value of real estate in the district increases by 4 percent annually in the future, the tax rates will exceed the $60 limits starting in 2014. District estimates show the rate for one of the bond measures will be in the $70 range from 2017-34, and in the $80-90 range for the other from 2020-2034. And that's all for repayment of bonds that have already been issued.

So how can the district charge more than it promised and issue additional bonds this fall?

First, the tax-rate limits are not binding. The law only requires that districts provide estimates at the time they issue bonds showing they will meet the limits. They are free to make rosy assumptions about rising real estate markets. Not surprisingly, many districts, including West Contra Costa, do just that. If assumptions don't pan out, bonds still must be repaid. So districts are free to raise the tax rates beyond what they originally estimated.

Second, the tax-rate limits apply individually to each bond measure. Thus, while the West Contra Costa district is on a trajectory to exceed the limits for the bonds approved in 2002 and 2005, it has not issued any of the bonds voters approved in 2010. That's the pot from which it will draw for the fall bond issue. Of course, it's highly doubtful voters would have approved the 2010 bonds if they had known the district would likely renege on its previous tax rate promises.

The district almost exceeded the limits this year. Because of favorable bond rates, officials were able to recently refinance the 2002 bonds and use the savings to hold rates down for two years. But the district can't count on being able to repeat that. To avoid exceeding the limits, the district could also refinance into longer-term bonds, but that would mean higher costs in later years for taxpayers.

The reality is the district has maxed out -- and will soon exceed the limit on -- two of its charge cards. Meanwhile, it plans to just pull out the next one.

Daniel Borenstein is a staff columnist and editorial writer. Reach him at 925-943-8248 or dborenstein@bayareanewsgroup.com. Follow him at twitter.com/borensteindan.