Fairfax City assistant manager Robert C. Norris has proposed an election-year budget for fiscal 1983 that would increase the city government's spending by 7.5 percent but would lower property tax rates by five cents.

In his budget message, Norris called the $23.4 million fiscal plan "conservative" and said in an interview that the budget provides for "a reasonable level of service" for city residents, who in May will elect a mayor and six council members to two-year terms.

City spokesman Thomas Welle said that while the spending plan, which must be approved by the council, would cut the property tax rate to $1.19 per $100 of assessed value, the owner of the average single-family house in the city would pay $68 more in property taxes because of higher assessments.

Concerned that the proposed budget might be viewed as a prelude to higher taxes after the election, City Councilman Carl Hemmer said he and his colleagues plan to "take a hard look at the budget" to avert speculation about such a scenario. The council has scheduled its first hearing on the budget for March 16, and is likely to approve the budget by April 13.

"I think there is always a temptation in election year to make the budget look as good as it can," said Hemmer. "What I'm very much afraid of is that we are going to see the New Federalism and Reaganomics raise costs without raising our revenue. We are going to have to look beyond this year to make sure we just aren't postponing tax increases," he said.

The city could boost spending $1.6 million over the current $21.7 million level without a tax increase, said Norris, because of its strong economy. He also said escalating property values will produce more revenue, even with the proposed five-cent decrease in the tax rate.

"To tell you the truth, the city has been lowering its tax rate for the last seven years, so it's not a one-time phenomenon," said Norris.

No new programs are proposed in the $23.4 million budget, but it recommends a 5 percent pay increase for city workers, who last year received an 8 percent cost-of-living raise. A 5 percent increase next year would cost the city $517,000. It also asks that three civilian police dispatchers be hired, as part of a "stepped-up crime prevention effort."

Other key budget proposals are:

* An $11.1 million outlay for schools, the largest single budget item. About $10 million of the sum will be paid to Fairfax County, which operates Fairfax City's schools.

* The transportation budget will rise 26 percent--from $411,221 to $518,857--to cover increased costs in operating Fairfax City buses and paying for the Metro bus subsidy.

* The mayor will receive a salary increase, from $5,000 to $6,500 annually, and city council members' pay will increase from $2,400 to $4,500 a year.