The Alexandria city manager proposed a city budget yesterday calling for 9.1 percent more spending and a 5 percent pay raise for municipal workers without increasing the real estate tax rate.

Manager Douglas Harman's proposal to the City Council relies largely on revenues resulting from increased real estate assessments to avoid raising the property tax rate, but calls for a new $24-a-year fee for trash collection.

The increase in assessments means that taxes for property owners will continue to rise. Overall, real estate assessments have risen 16.5 percent in the last year, with assessments for single-family homes and condominiums rising an average of 7 percent and apartment developments and undeveloped commercial and industrial land rising 40 percent.

Much of the disparity in assessment increases is due to state law that requires apartment houses to be reassessed annually at fair market value, or essentially at what such a property would be worth if it were a condominium. Harman estimated that the apartment reassessments will yield $2.3 million in additional revenue for the fiscal year that starts July 1.

The trash disposal fee, which would be assessed on each dwelling unit, would pay for maintenance of landfill sites and is necessary, Harman said, because of expected decreases in federal revenue sharing funds.

Harman's $134.1 million spending proposal includes the school board's $46 million budget request.

"There is no funding safety net included in this budget," Harman said. "If there are major cuts in this fiscal year, this budget doesn't have the flexibility to deal with them." He warned that the President Reagan's federal budget proposal calls for major reductions and elimination of dozens of programs that would seriously effect Alexandria. The city's contingency fund has been $200,000 for the last decade; it would not be increased in the manager's budget.

One major area of funding uncertainty is Metro. The council has indicated that its contribution to the area transit system will remain at its current level of $5.7 million. There is an additional $800,000 reserve available in a general transportation fund, but Harman said yesterday that even $6.5 million is "less than what can reasonably be expected" to be the cost of transit service to the city. Latest transit agency estimates of costs during the current fiscal year are at $7 million, which means Alexandria already is in debt.

Harmon's recommendations include 24 new city jobs, many of them in the area of social services. The proposed budget includes $149,600 to fund extended day-care services and staff for the city's battered women's shelter.