A financial consultant's study underestimated the amount of money Montgomery County can borrow to pay for roads, schools and other building projects, county officials said yesterday. The error meant that estimates of the county's debt capacity was about 10 percent too small.

The study by a New York consultant was used as one justification for restricting spending on a number of projects and has become a central issue in recent debates over how much the county should spend on capital improvements over the next six years. Much of that money is borrowed by issuing bonds.

County Executive Charles W. Gilchrist, who has proposed a $1.03 billion budget for the six-year capital spending plan, has argued that the county would risk its valued AAA bond rating -- the highest credit rating a county can have -- if it started borrowing more. The rating means that the county can sell its bonds at a low rate of interest and is viewed as a sign of its economic health, valuable in attracting new industry and investors.

Council members have argued that Gilchrist's proposals were under-funded, and that that the county would have to borrow more money or eliminate planned projects. School officials, faced with growing enrollment, have argued that the county needs to spend more money than Gilchrist proposes on school building and renovations.

Yesterday, Gilchrist acknowledged a $48 million error had been made in the study by the First Boston Corp., but said it would make no difference to his hotly debated capital improvements plan. "Our budget is what we think is needed," he said. The added borrowing power, he added, "gives us a little more flexibility. It doesn't suggest any changes."

County Finance Director Max R. Bohnstedt called the error "serious" but said be believes the study was a good one. Regardless of the correction, he said, the county remains at the outer limits of its borrowing power. Bohnstedt said he discovered the error, part of a complex mathematical formula, a week ago and informed First Boston, which sent representatives to Rockville this week.

First Boston Vice President Arthur Schloss, in a letter to Bohnstedt explaining the error, said he believed that the central conclusion of the study, that the county should closely base its bond issues on its tax base and budget growth, "remains sound." He said "a conservative approach to debt issuance must be pursued."

The County Council is scheduled to discuss Monday night how much money should be spent on capital improvements, and how much of it should be raised by bonds. The council's finance committee voted unanimously on Thursday that, however large the budget, it should not risk the AAA bond rating.