A tunnel, bridge and possbily solar panels are all part of the plans to double the size of Washington's National Presbyterian School. But the $1.5 million addition will look just like the present fieldstone and stucco structure that faces Nebraska Avenue, according to architect Harold Wagoner.

"When it's finished, you'll hardly be able to tell they weren't built at the same time," Wagoner told a group of nearly 100 parents, church members and teachers assembled in the school chapel to see the plans.

Scheduled for completion by January 1981, the 35,000-square-foot addition will enable the school to add fourth, fifth and sixth grade classes. The school, which opened in 1969 at the same time as the modern gothic-style church, presently offers nursery through third grade classes.

"We want our children to go to fine schools and it's increasingly difficult to find spaces for them at the fourth grade level," school principal Jane Harter said. "Our third graders want to stay together. It's important for them to continue their educational experience with friends. Moving to another school can jar their development. The big family atmosphere we have here is a definite asset."

Wagoner used slides and schematic drawings to show how the new two-story building will be built directly onto the rear of the present school. Because of the landscaping, there will be a bridge connecting one entrance to the church courtyard.Beneath the bridge will be a tunnel. Wagoner also said that the building's roof would be ideal for solar panels and he is studying possible grants to help fund the alternative energy source.

The English gothic style school building is one of four buildings that housed an orphanage before the church purchased the property in 1967. Wagoner, who designed the modern church in the center of the orphanage complex, said that he at first had wanted to tear down the 1928 buildings but decided against it because they were so well-built. He said the gables and fieldstone gave the school a friendly domestic atmosphere that he was eager to preserve.

Although the school is under the aegis of the National Presbyterian Church, its 150 students, whose families pay $1,700 a year tuition, come from many faiths. While children read Bible stories and say prayers, there are presently no formal religious classes. These would be added for fourth through sixth grades.

"The only thing that stands between us and the expansion of the school is nasty, dirty, old money," development committee cochairman George Gaines told the group. He said the committee hoped to raise as much of the $1.5 million as possible by October. The committee can then determine whether the 1981 goal for completing the school is practical.

Financial consultant John Brown told parents and church members about planned gift-giving and other aspects of fundraising, saying, "I'm here to stimulate your thinking. There are resources you can all go to in your spheres of influence - even relatives."

Gifts and donations are the chief ways the development committee, headed by Phyllis Clark, hopes to raise the money. The effort hasn't really started yet but, according to Clark, "We've got some real good nibbles."

Parents who stayed for petits fours and sandwiches seemed optimistic and pleased about the plans. "Actually, we were surprised at how modest the cost is," said Brendan Keegan, whose son and daughter think about it, there are homes in D.C. that cost a quarter of a million." CAPTION: Picture, Architect Harold E. Wagner explains plans for a two-story addition to the National Presbyterian School. By Craig Herndon - The Washington Post