A day in a Northern Virginia hospital costs an average of $176, double what it was five years ago.

At the same time, a typical type of non-group health insurance coverage supplied by Blue Cross-Blue Shield will soon cost subscribers in Northern Virginia $71.40 a month, compared to a monthly rate of $29.43 five years ago.

Rising costs like these, health specialists say, are partly due to overexpansion of hospitals, sophisticated medical equipment and medical services. One way to curb the overexpansion, they add, is planning - planning that would look at existing medical facilities throughout the metropolitan area before building more.

Northern Virginia has taken a step toward regional planning that would look beyond its own borders by signing an agreement with two neighboring jurisdictions to prevent unnecessary duplication of hospitals and other health services.

The Northern Virginia Health Systems Agency, the federally mandated health planning organization for suburban Virginia, recently signed the agreement with the District government and two sister health planning groups in suburban Maryland.

The agreement calls for all the health planning groups to join forces to combat overexpansion of health facilities throughout metropolitan Washington. Blue Cross officials call the regional planning approach "the only alternative to enormous waste and expense."

"This agreement will give health planning agencies a much stronger stance, a more effective role in health planning," said Dean Montgomery, director of the Northern Virginia Health Systems Agency. "It will provide an open forum to look at health planning on a regional basis."

All the area health planning organizations will continue to make decisions concerning health care in their own jurisdictions.

But representatives from each of the agencies, according to the agreement, will exchange reports and views and invite each other to sit as non-voting members when the organizations examine new requests for medical buildings and equipment. The Northern Virginia Health Systems Agency reviews medical project proposals costing more than $100,000 and advises state health officials on whether they are needed in Northern Virginia. The state health commissioner ultimately decides on the construction or purchase of medical facilities.

"The agreement also calls for periodic meetings among representatives of the agencies," Montgomery said. "But we will probably try to set up monthly meetings to keep up a strong crosscurrent of information."

Currently there is no formal exchange of information or coordinated effort among the area's health planning agencies to control the growth of new hospitals, sophisticated medical equipment or expensive services in the region.

Blue Cross analysts project that at the present rate of construction, the metropolitan area will have more than 1,170 unneeded hospital beds by 1980, which will cost $70 million a year to maintain. Blue Cross maintains that the time for regional efforts to control health care overexpansion and the consequent higher costs is long overdue.

The agreement not only provides for the exchange of infomation in frequent inter-jurisdictional meetings, but also sets up a data base compiled from information gathered by the individual agencies. It also calls for the development of common positions on health planning issues and the establishment of criteria to identify health facilites or plans that would affect more than one jurisdiction.

Under the criteria, proposed health programs or facilities would be considered to have a metropolitan-wide effect if a significant number of people crossed jurisdictional boundaries in order to use them; if they affect the delivery, cost or quality of services in another health service area, or if the proposed service is highly specialized (e.g. heart surgery and kidney care) and requires regional coordination to insure high quality and efficient utilization.

The agreement comes at a time when each of the health planning agencies is developing comprehensive plans for health care growth and distribution in their jurisdictions.

Input from the individual agencies in each other's plans hopefully will produce common positions on health planning issues, a Northern Virginia health planner said.

The development of a cooperative health planning scheme for the entire metropolitan area also comes three years after Congress passed a new national health planning act that set a goal to establish strong regional planning bodies to cut across state lines in multistate metropolitan areas.

At various times. District, maryland and Virginia officials refused to join in a single agency. The new agreement, while not creating one umbrella agency for the region, does form a common ground for the separate groups to work toward the same goals - keeping health care facilites well-distributed and well-utilized while curbing the rising costs of health care.