The Navy's Trident submarine, the world's largest, is at least $400 million over its estimated cost and a year behind schedule, admirals in charge of the program said yesterday.

The 50 per cent cost ovrerun - from about $800 million to $1.2 billion for the first Trident under construction by Electric Boat Co. in Groton, Conn. - involves the hull, not the missiles or nuclear power plant that go inside the 18,700-ton submarine.

Rear Adms. Albert L. Kelln and Donald P. Hall, Trident program directors, told reporters at a Pentagon news conference that they could not estimate offhand what the total cost of the first Trident would be.

But, since each of the 24 nuclear-tipped missiles to go inside the Trident costs about $10 million and the Navy will have to build a new shore complex to accomodate the 560-foot-long submarine, the ultimate cost overrun could top the record $2 billion experienced on the Air Force C-5 transport plane.

Hall said the Navy's current estimate is $22 billion for 13 Trident submarines, with $13 billion of that total earmarked for the submarines alone and the balance for research, weapons and shore facilities. The Air Force's B-1 bomber project that President Carter canceled would have cost an estimated $24 billion.

"This is a complex, very large ship," Hall said in trying to explain the cost overrun of $400 million on the first Trident, which is now expected to be delivered in 1981 rather than 1980. "We've never put together" a submarine "of this size before."

The admirals conceded their problems are further complicated by the fact that Electric Boat has been laying off workers while the only other shipyard that is capable of building Trident has declined to help build it.

That yard is Newport News Shipbuilding, a Tennico company in Virginia, which is fighting with the Navy over who owes whom for its work on nuclear-powered cruisers. Its expressed disinclination to build Trident is an example of the poor relations between the Navy and its shipbuilders.

The controversy started before any Trident contracts were let, however, at Pentagon critics argued that it would not be worth building such a big submarine to replace the fllet of 41 Polaris and Poseidon boats, each of which carries 16 missiles. But Adm. H.G. Rickover, director of Navy nuclear propulsion, and his allies won the argument.

What forced the Trident to be so huge, one critic said yesterday, was Rickover's insistence that the power plant be big enough to drive the Trident at 25 knots while submerged. The hull had to be designed around this requirement. The first generation of Trident missiles are little bigger than Poseidon missiles carried on the smaller boats.

Even if the Trident does manage 25 knots, the critics argued, Soviet killer submarines that steam over 30 knots could still overtake the missile sub. Also, it was argued, Trident's bulk would provide a huge reflecting surface for the searching sound waves that Soviet killer subs might send out rather than settle for listening for the enemy.

One alternative proposed to save money called for building a sub that would be so strong that it could rely on depth - moving along just off the ocean floor - rather than on speed to escape detection.

Another rejected concept was to build a new fleet of missile submarines smaller than Trident and based at several ports so that there would not be so many nuclear eggs in one basket. This plan, one Trident critic said yesterday, would have avoided the danger of tomorrow's missile submarines of the Trident class, all using the same port in Bangor, Wash., where the Soviets could keep track of them.

Congress so far has gone along with the Trident program, although some members have warned against putting a first-strike type of missile in the new submarine.

The Navy credits the initial Trident missiles with a range of 4,000 nautical miles compared with at least 2,500 nautical miles for the Poseidon inside submarine now on patrol.

The longer reach of Trident I will enable the submarines to cover more of the Soviet Union while patrolling farther from its shores.

Trident is not a first-strike weapon system," Adm. Kelln said yesterday, adding that the new submarine force will "decrease any incentives" for attacking the United States by serving notice retaliation would follow from nuclear weapons hidden in the ocean depths. "Trident will contribute to worldwide crisis stability," he said.

The Trident submarine tubes are large enough to accomodate a much larger missile. Rep. Thomas J. Downey (D-N.Y.), a member of the House Armed Services Committee, is among those in Congress warning against deploying the Trident II nuclear block-buster still on the drawing board.

He argues that the Soviets would regard a larger and more accurate Trident missile as a first-strike weapon threatening their bomber and missile basses. The Soviets would follow suit. Downey contends, making the current balance of terror more precarious. CAPTION: Picture, The Navy plans to build 13 nuclear-powered Trident submarines, each carrying 24 missiles, for $22 billion. The Washington Post