Apartment owners at the Promenade cooperative in Bethesda have been surprised with bills of between $400 and $1,200, due on March 25, for property taxes at the controversial conversion.

Unit owners say they are angry because tax costs are supposed to be included in their monthly payments they make, and no adequate explanation has been given by American Invsco, the financially troubled apartment converter and owner of the Promenade project, on why the special assessment was necessary. In addition, apartment owners point out, American Invsco has not made any payment on the property taxes, which were due in September.

A spokesman for American Invsco said, however, that the special assessment is directly attributable to the sharply increased tax bill on the complex from the state of Maryland and Montgomery County. The property tax was increased from $469,000 for fiscal 1981 to $1.4 million in fiscal 1982, which began last July 1.

Since then, American Invsco has contested the amount and has succeeded in getting the bill reduced by $250,000. The converter continues to contest the amount, however, and that is why the tax still has not been paid, said David Varner, a spokesman for American Invsco.

In the meantime, all the regular payments that have been made monthly by unit owners for the taxes have been put in an escrow account, to be used when the final tax bill is finally paid, Varner said.

The special assessment, sent to apartment owners the week before Christmas, is in addition to a 35 percent increase at the beginning of the year in the monthly payments that owners make for an underlying mortgage, which is to include tax payments, and a 9.1 percent rise in maintenance fees, according to James Nickelsporn, former president of the tenants association at the Promenade and now a unit owner.