The surprising vote Tuesday to modify Prince George's County's tight lid on property taxes is expected to add $30 to the average homeowner's bill the first year and between $6 million and $8 million for services to the county's coffers. But its supporters say that it still may not be enough to fill the county's projected revenue gap next year.

The amendment to the 6-year-old tax-limiting initiative known as TRIM (Tax Reform Initiative by Marylanders) will go into effect in fiscal 1986, which begins in July. Since county officials are in only the preliminary stages of charting that budget, the effects of the modification are still unclear. But County Executive Parris N. Glendening has promised that the added revenues will be funneled into the fire and police departments, as well as schools in response to growing concerns about the county's education system.

However, the difference, he said, will mean only that there will be fewer cuts in those departments, not that there will be additions.

"It stops the deterioration," he said of the measure, which also freezes the county tax rate at $2.40 per $100 assessed value, three cents below the current level. "But it is not a large source of new money."

TRIM, the strictest tax limit in the country, had imposed an absolute $143.9 million ceiling on property tax revenues. The change, although it lowers the current rate, will allow property tax levies and collections to rise as assessments increase. Officials of FACT (Fairness for All County Taxpayers), the group that supported the charter amendment, have estimated that the change will cost the average taxpayer about $30 during its first year.

Far more valuable than the increase in revenues, Glendening and others said, is the positive message that passage of the modification sends to developers who are interested in Prince George's and to the Maryland General Assembly, which has the power to give the county more taxing authority.

"The financial success is one thing, but the bigger victory was in internal morale . . . ," Glendening said yesterday.

Glendening's top aide, Chief Administrative Officer John Wesley White, said that preliminary fiscal year 1986 budget projections already have begun to show that even a TRIM change cannot cure the county's budgetary problems.

He said that unless the county "comes up with alternative strategies" in addition to the relaxation of TRIM, county departments will face "substantial reductions."

"The proceeds from TRIM modification will not actually even keep pace with the escalated costs we're predicting," White said.

School Superintendent John A. Murphy said that any extra money diverted to his department will go toward reducing class size by hiring more teachers. But the money from the TRIM change, he added, probably will be just enough to ensure that class sizes will not increase.

Police Chief Michael J. Flaherty said that his department will just be plugging holes that already exist with any new money coming its way.

"If it hadn't passed," he said, "my basic concern was how we were going to continue to provide basic police service without cuts."

FACT workers and county officials said they were caught off guard by the size of the positive vote in Tuesday's election. Final unofficial figures show that voters approved Question A by 78,719 to 68,193.

TRIM coauthor David Bird, a state delegate from Cheverly, said that his supporters were simply outspent. FACT raised about$45,000 to TRIM's $3,000.

"Ultimately, you have a concern about the schools, and a feeling of frustration that maybe money can solve the problem," Bird said.

Gerard Devlin, a state delegate from Bowie, an area that voted for TRIM in 1978 and spurned a 1982 amendment effort, said that black and liberal white voters banded together to outweigh the strength of conservatives who opposed TRIM changes.

"The case was made," Devlin said. "People in this area of the county are very interested in education. Polls in my district have always showed that people are willing to spend more money on education."

Heavy voter registration and turnout in black precincts also worked to Question A's advantage, according to FACT supporters and Rainbow Coalition workers.

"TRIM would not have made it without us," said Bennie Thayer, the state chairman of the Rainbow Coalition. "We had to deliver on that one.