Montgomery County may have to increase its property tax rate by about 7 percent next year to keep county services at their current level, according to an analysis by a top county fiscal officer.

The increase would add about $80 to the average tax bill of approximately $1,200 paid this year by the owner of an $80,000 home, assessed at half that amount for tax purposes.

County Council fiscal officer Arthur Spengler said the possible 7 percent rise takes into account projections for inflation wage increases for county workers, proposed county subsidies for the Metro system and expected county revenues in fiscal 1981, which begins next July.

The 7 percent increase would yield about $14 million and allow the county to pass a budget of $623 million -- up about 15 percent over this year's budget.

After seeing the analysis, the council directed its staff to develop three alternative budgets, including the one for $623 million. Another alternative -- considered a highly unlikely candidate because it does not allow for any inflation -- would be pegged to the current year's budget of $569. A third falls in between and relies on some increased revenues, but does not call for a tax rate increase, according to Spengler.

The budget analysis, which Spengler stressed is "only a guideline," is based on several monetary pluses and minuses the county expects to face the coming fiscal. year.

On the minus side, a 10 percent increase in wages to county employes would cost $40 million, a proposed increased Metro subsidy increase would cost $5 million, and an estimated 11 percent inflation rate would add another$11 million to the budget needs, according to Spengler.

On the plus side, the county could save about $6 million by keeping school spending down because of a projected 5 percent enrollment drop next year, according to Spengler. Reassessment of one-third of the county's taxable properties also will yield added revenues. But because the state has placed a ceiling on annual assessment increases, they will not reflect the soaring property values Spengler said. Last spring, in response to Proposition 13 style property tax protests, the county cut the tax rate by 36 cents. A 7 percent rise would add 20 cents to the current rate of $3.08 per $100 of assessed valuation.