Plans for the Sea Colony North condominium development near Bethany Beach, Del., were blocked for a second time in seven years Monday by the Delaware Supreme Court.

The court voted unanimously to invalidate rezoning ordinances approved by Sussex County in 1978 and 1980 that permitted Carl M. Freeman Associates to construct a 432-unit project of three high-rise buildings on 24 acres of ocean-front property north of the town.

Freeman, a Washington area builder, has already constructed two major resort complexes, Sea Colony East and Sea Colony West, south of Bethany Beach. The developments include eight high-rise apartment buildings and several hundred town houses.

The suit blocking the third complex was filed by property owners in the tiny beach town, which is sandwiched between the existing Sea Colony developments and the site of the proposed complex. They contend that the fragile seacoast environment cannot support another high-rise apartment development and that roads and sewer lines to the proposed complex are inadequate.

The Delaware high court struck down the county zoning ordinances on procedural grounds, upholding lower court findings that the county failed to adequately publicize hearings on the proposed zoning changes.

The decision is a victory for Bethesda attorney William Green and several other Bethany Beach homeowners who have tried to block the Sea Colony North plans for a decade.

Although the court's decision ruled against Freeman and Sussex County, the fact that it is based on the county's procedural mistakes rather than the merits of the project still leaves open the possibility for development on the site, Green said. He added that he will oppose any new development plans, however.

In the written decision, the state high court's chief justice, Daniel Herrman, said that the Sussex County Council made a "fatal" mistake when it advertised Freeman's application in only one newspaper. Delaware law requires advertisements to be placed in all newspapers in the county, in this case a total of six.

The court also struck down a "curative ordinance" passed by the council in 1980 that was designed to correct the improper advertising. The court said this ordinance should have also been considered by the county Planning and Zoning Commission.

In striking down the "curative ordinance," the court's ruling applies only to the Sea Colony application and will not affect approximately 340 other zoning changes made in the county under the now invalid law, according to acting Sussex County counsel Dennis Schrader.

The Sea Colony North proposal came first before the Delaware Supreme Court in 1975, three years after Freeman first filed plans with the county. The high court ruled that the first proposal did not meet legal specifications for adequate sewerage and public access.

Schrader said another application to rezone the land for high-rise apartments or condominiums would have to "go back to the beginning of the planning process and start over again."

The Associated Press reported that Charles W. Cole, one of two county councilmen opposed to the rezoning, said he would not be surprised to see Freeman reapply, but that conditions may have become less favorable for the developer.

"The way I look at rezoning," Cole said, "it's a case of proving hardship."

Development of a smaller project would be no special hardship for Freeman because such housing is selling well, Cole said.

Freeman's attorney, John A. Sergovic Jr., of Georgetown, Del., refused comment on the decision.

A spokesman for Freeman Associates could not be reached for comment.

Future development of the 24-acre site may be considerably more difficult if the Interior Department is successful in designating the Bethany Beach area as a "coastal barrier."

Under a law passed by Congress last year, the Interior Department may make certain fragile coastal areas ineligible for flood insurance. While development would not be prohibited, the effect of designating an area like Bethany Beach as a coastal barrier would be to make any substantial development extraordinarily risky.

The Bethany Beach site is one of 159 coastal parcels under consideration to become a barrier, according to Hardy Pearce, staff director of the Interior Department's coastal barrier task force. If it is approved, property on Bethany Beach would become ineligible for flood insurance on October 1, 1983.