Even more than most movie studios, Elstree, just northwest of London in Hertfordshire, looks like an inhabited ruin.

Its streets are narrow, muddy and cluttered and its buildings old and well-scarred, somehow suttesting that the impermanence of false-front sets had proved contagious and spread, like measles.

But amid the busy jumble, crowning a slight rise of land like a dark castle, stands a huge and brand-new sound stage, built by George Lucas under an arrangement with EMI, which owns Elstree, for the making of "The Empire Strikes Back," as "Star Wars II" is being called.

It is second in size only to the huge James Bond stage at Shepperton.

The new Elstree stage, on which football could be played, has on a late spring morning been frosted inside, so to speak, its cavernous interior decorated to resemble an enormous ice cave.

This is on the ice planet Hoth, where our friends of the Rebel Alliance (Harrison Ford, Mark Hamill and Carrie Fisher) and their warriors have made a temporary headquarters in their grim galactic struggle against the wicked Empire.

These interiors will match up with location footage shot in fierce weather conditions on a glacier at Finse, between Oslo and Bergen in Norway, the same spot at which Robert Scott trained for his ill-fated assault on the South Pole in 1910.

At temperatures of 15 below zero (and a wind-chill factor many degrees lower than that), cast and crew shot chases and struggles in the blinding snow, battles involving men, machines and large, llama-like but imaginary beasts called tauntauns, which are one of the several inventions of these further "Star Wars" adventures.

Just now, at Elstree, however, our friends are fleeing Hoth. The stage, its floor covered with salt to resemble snow, is full of vehicles - futuristic personnel carriers and small fighter craft called Armoredspeeders (in the "Star Wars" lexicon), which resemble snowmobiles with guns.

A large population of extras, some in white space suits, some in orange, are poised to run their vehicles, cheering. A technician walks about laying down a thick curtain of smoke. Brilliant light at the far end of the stage suggests the snow plain outside.

"All civilians off, thank you, ready or not," an assistant director says politely through a bullhorn. Director Irvin Kershner and producer Gary Kurtz stand atop scaffolding, watching alongside the principle camera. Hamill, as Luke Skywalker, is the only star in the shot. In all his gear, he looks like an electronic toy.

The cry of "Action!" produces a stampede of movement, the speeders pushed across lens-view by unseen poles.

Three years ago, at the same studio, "Star Wars" was a rather mysterious project, proceeding under tight security but with a kind of low-keyed family intimacy. Producer Kurtz and writer-director Lucas were worried whether it was possible to do all they wanted to do on a budget of only $8 million.

The final direct cost turned out to be $10.5 million, which in turn proved to be one of the great bargains in film history. The last time Kurtz looked, "Star Wars" had earned about $65 million in rentals in the United States and Canada and something more than $70 million abroad. Customers have paid presumably something on the order of a half-billion dollars to see "Star Wars."

There will be a wide brief re-release of the film later this summer. The end is not in sight.

"The Empire Strikes Back" is budgeted at about $18 million, a modest enough figure, particularly since this time sets and props are being constructed with further films in mind. Had anyone guessed just how successful "Star Wars" would prove to be, Kurtz and Lucas & Co. could have built the first time for the long run. Even so, this production has been able to recycle some of the sets and props acquired the first time, as well as add new, permanent hardware, like the R4D5 droid.

This time, the idea is that there will indeed be a tomorrow. The Millenium Falcon, the spaceship in which our trio of heroes does its flying, was designed and rebuilt like a giant Erector set in 16 sections for ease of assembling, cutaway shooting and storage. It's 60 feet across (80 with its probe, or mandible) and made of steel at the former Royal Navy shipyard in Pembrokeshire, Wales; it weight 40 tons.

"The Empire Strikes Back" sprawls over eight stages at Elstree, but the sets are so numerous and elaborate that striking one complex set and raising the next one in time to meet the shooting schedule is an impressive exercise in logistics.

While the good guys were evacuating Hoth, crews on another stage were rushing to finish a spectacularly large, hanging black dome called the carbon freezing chamber, complete with a monstrous lifting claw and sufficient pipes and gizmos and sources of steam and smoke and lurid light either to solve the energy crisis or complicate it beyond any hope.

On another stage, a swamp planet was taking shape. Men were creating an extraterrestial forest, smearing branches with hands dipped into battered dishpans full of brown polystyrene goo.

Altogether the "Empire Strikes Back" team numbers more than 400, here and back in Los Angeles and San Francisco, where the special effects will be made and mated to the principal photography.

George Lucas himself, the first and only begetter of "Star Wars," is executive producer this time, a visitor commuting from California, where he has been putting the last touches on "More American Graffiti."

The story is his, the second helping of what he envisioned from the start as a trilogy called "The Luke Skywalker Legend." The first draft of the script of "The Empire Strikes Back" was completed by Leigh Brackett just before her death, with the final version done by Lawrence Kasdan.

Kurtz says that on paper there are the makings of a grand design of 12 films, including three stories that would historically precede the Skywalker tales ("prequels," as they are dreadfully known these days).

How much of Lucas' long vision is translated into film depends, of course, on how avid the market stays. There is no sign yet of any slackening of interest in science-fiction fantasy, as the fast start for "Alien" appears to confirm.

"It will be interesting to see," says Kurtz, "when "Star Trek," "The Black Hole," "The Shape of Things to Come," "The Martian Chronicles' and "Brave New World" all start coming out in the next several months." "Empire Strikes Back" has a shooting schedule of 19 weeks and is set for release in the summer of 1980, on the third anniversary of "Star Wars."

There will be less for background exposition this time. (Alec Guinness will be on hand for a very brief visit, no one is saying in just what context, though a flashback seems reasonable.)

You need only a quick look at the razzle-dazzle sets to know that vigorously combative and colorfully located action will be, as before, the major imgredient. But Kurtz says there is in fact more attention to character, including a certain amount of romantic tension in the triangle of Harrison Ford as Han Solo, Hamill as Luke Skywalker, and Carrie Fisher as Princess Leia.

The push on character development is one of the reasons the choice as director fell this time to Kershner, whose previous works ("Loving," "The Eyes of Laura Mars" among them) have been far from this genre but heavy on character.

"It was important," Kurtz says, "to have a director who believed in the fantasy. You couldn't be cynical about it. That would be fatal. Irvin loves the genre, although he'd never done a film in it."

"Empire Strikes Back," like "Star Wars," will reflect the visual inspirations of Ralph McQuarrie, a quiet and shy English-born illustrator who studied at the Art Center College of Design in Los Angeles and worked as a technical illustrator for Boeing and others, before he found his way to his real calling as a visual consultant for movies. He is now a full-time Lucas employe, and the world of tomorrow (or whenever) is imaginatively his. CAPTION: Picture, no caption