The Environmental Protection Agency is warning Washington-area localities that they can no longer count on the federal government to subsidize construction of sewage treatment plants.

The message EPA is sending out - here and elsewhere around the nation - is that the agency will fund only projects that provide for a moderate amount of growth.

Localities that want to build treatment capacity for more growth will have to pay the total cost themselves, through higher taxes or higher sewerage fees under EPA's tougher policy. Localities that want EPA funding may find it necessary to take a more activitist role in controlling growth within their boundaries.

EPA has spent or committed itself to more than $1 billion for Washington area treatment plants - all of it, obstensibly, to clean up the polluted Potomac. Under the Clean Water Act, cleanup of polluted waters has the highest priority for EPA funds.

"The simple fact is there is not enough money in the federal program to fund every possible project at the absolute size that a community wants it funded," one high official with EPA's regional office said.

The new EPA policy already has had an impact locally. Last month, EPA told Montgomery County it would not fund what the county says it will need from about 1984 to 1993 - another 20 million gallons worth of daily treatment capacity. EPA said it might subsidize about one fourth of the planned expansion.

As a result, some Montgomery leaders are urging that the county finance the difference. The cost could be more than $50 million.

Fairfax County has decided to spend $21.5 million to finance a cross-county sewer pipeline if EPA does not provide 75 percent aid.

Fairfax's next big project is to be a 50-percent expansion of its lowr Potomac treatment plant in the mid-1980s. The county has already begun paperwork for an EPA grant for the project. If EPA turns down the application because Fairfax's growth rate - by far the highest in the area - is too rapid, the county would be hard-pressed to raise the money.

Until now, EPA has rarely told localities it thought they were building facilities that were too big or otherwise not economical. The result, EPA officials countered, has sometimes been overbuilding.

At the present time, suburban Maryland has unused sewer capacity that could serve more than 200,000 additional people, but a lot of it is not in areas where it is needed.

EPA officials say Montgomery might be able to avoid building some new treatment plants by getting better performance from its present system.

Localities have not had much incentive in the past to take these steps.But the alternative - paying for costly new treatment plants by themselves - may provide that incentive some EPA officials say.