The estimated cost of building Washington's Metro to its planned 100-mile length would be about $5.5 billion, or $420 million more than the last best estimate of six months ago, the Metro board was told yesterday.

The Metro construction staff, in its semiannual review of costs and schedules, also told the board that contractors were encountering unusual difficulty in completing an extension of the Blue Line from Stadium-Armory to Addision Road in Prince George's County. That time is now projected to open in middle to late 1980 instead of November 1979.

Other projected completion dates have slipped, most notably on the four Metro lines that are being restudied under federal order. The next line to open will be the New Carrolton line from Statium-Armory, the first segment of the Orange Line to open. That opening still is scheduled for next November.

The Metro staff's official new total system construction estimate is $5.188 billion, but that total does not contain $593 million in contingencies Metro staff members, when pressed by board chairman Joseph S. Wholey, acknowledged that two of those contingencies are very likely to occur.

"If I moved those two contingencies to the total column, would that be the best estimate?" Wholey asked Donald R. O'Hearn, Metro's director of program control.

"I think that it would," O'Hearn said.

The two contingencies thus added to the total cost are $215 million for 218 more cars to run on the 100-mile system, and $56 million caused by "critical decision delays." That is bureaucratese for the federally induced added cost of restudying the uncompleted four lines.

Of course, if the final result of that study is to cut back the Metro system, the total cost would be proportionately less. It is, nonetheless, now considerably more than double the $2.5 billion Metro was supposed to cost when the general plans for the system were nailed down in 1969.

The rail car issue has been emerging as a result of the regional restudy in the past months. Metro was showing a total need of 482 cars for 100 miles. Peat, Marwick, Mitchell and Co., the consultant to the regional task force that is restudying Metro, had concluded that 700 cars was a more likely number.

"We just had to start showing additional cars, Metro general manager Theodore Lutz said late yesterday.

The cost of rail cars is getting out of sight, to the great concern of transit operators generally. Metro's initial order of 300 cars was built by a Rohr Industries subsidiary for $300,000 a car. Rohr lost millions on the contract and no longer builds cars.

Atlanta's new subway, scheduled to open in December, purchased its cars from a French manufacturer for $560,000 each "and the final cost may be more," a Metropolitan Atlanta Rapid Transit Authority spokesman said yesterday.

Metro is projecting that it will pay $750,000 to $850,000 a car for its next order. The 218 cars included in the contingency are projected to cost about $1 million each.

Rail cars do have the virtue of lasting for 30 to 40 years. No bus can make that statement.

Setting aside the two contingencies, the big items, in the new total construction cost estimate are for insurance and changes in the construction contracts after they have been awarded.

"Change orders" have been running about 10.3 percent for tunnel work in Metro construction completed to date, about half that for other construction. The latest cost estimate projects $74.6 million for added insurance costs and $40.3 million for change orders.

The insurance cost is brought on by the high workmen's compensation Metro contractors must pay to employes who miss work because of job-related injuries. Under an administrative ruling all Metro construction workers - no matter whether they are working Maryland, Virginia or the District of Columbia - must be compensated at the higher District of Columbia rate.

Since 1976, the number of workers per hundred who have suffered a lost-time accident has increased from 6.8 to 8.2, according to O'Hearn. Since 1972, the maximum benefit that must be paid under the D.C. compensation rate has increased from $70 a week to $367 a week. Payments are based on a sliding scale and are tax free. By comparison, the Maryland rate is $188 a week and the Virginia rate, $162.

O'Hearn said that the 8.2 rate, while high for construction generally, was not high for Metro's particular type of construction. He said Atlanta's MARTA rate is the same as Metro's and that the San Francisco Bay Area Rapid Transit (BART) rate was 8.5.

Metro's insurance premium on construction is now $47 for each $100 of payroll for contractor employes, and $40 of that $47 goes for workmen's compensation.

As O'Hearn and Lutz rolled through the dreary news for the board yesterday, they also explained a new incentive program they are implementing to encourage contractors to run safe, injury-free operations.

Under the program, a contractor will be paid $20,000 for each saving of one-tenth off the 8.2 base that he achieves on a project (that would amount to 10 per cent of the savings to Metro in insurance premiums).

On the other hand, if the contractor exceeds the incidence rate of 8.2, funds will be withheld from his monthly payments from Metro.

"At the present time, we don't feel that the contractor has enough incentive," O'Hearn said.