Despite considerable citizen opposition, the Hyattsville City Council this week tentatively approved plans to expand the Washington Suburban Sanitary Commission's crowded administrative headquarters in the city.

Caught between WSSC threats to leave the city and citizen ire over the proposed expansion, the council reserved the right to reject the WSSC proposals during the planning stages if they do not conform to the city's environmental needs.

In a crowded public hearing two weeks ago, residents protested plans to construct a two-story, 60,000-square-foot annex on the WSSC-owned parking lot adjacent to the building on Hamilton Street. Citizens said they fear increased traffic on narrow residential streets and encroachment on the nearby Magruder Park. Residents who have fought previous WSSC expansion attempts said they were "disappointed" with the City Council decision.

"I believe their action was misguided," said James D. Riley, president of Concerned Citizens of Hyattsville. "There is no question on my mind that most citizens oppose the expansion."

Residents presented the City Council with a petition containing 263 signatures opposing the expansion. The signatures were collected in Ward 2, which would be most affected by increased traffic.

"There's been a longstanding opposition to expanding the building," said Spencer C. Hines, council member from Ward 1, who voted against the measures, along with the other Ward 1 representatives, DOUGLAS S. Dudrow.

WSSC officials said they were pleased with the decision.

"It's the most positive step the city has taken in 10 years," said WSSC assistant manager John M. Brusnighan. "In the past it's always been a flat rejection."

The WSSC has been trying to expand its four-story building since 1969 when officials first proposed a vertical addition on top of the building. Commission officials have studied and rejected the construction of a new building on Rte. 1 in downtown Hyattsville and recently have discussed the possiblility of relocating out of the city and perhaps out of Prince George's County.

Limited space in the present location on Hamilton Street has forced the WSSC to rent 13,000 square feet of office space in the Presidential building in Prince George's Plaza and 56,000 square feet in the Arbitron building in Laurel, at a cost $578,000 for both leases.

The proposal approved this week was introduced during a meeting in late October between City Council members and WSSC officials. Mayor Thomas L. Bass said he called the October session to verifty WSSC's plans and to explore the possibility of a compromise that will keep the commission in the city.