A walk-up apartment in the city is mod -- it's camp -- but a fifth-floor walk up is something else. For Chuck and Joni Mitchell, it is many things. It's a walk in the park... a browsing session in a library... a midnight view of the city... a stroll through an art gallery.

These are just a few of the reasons this young couple, appearing in Detroit area nightspots as a folk-singing duo, chose an inner city apartment.

For them, the setting is perfect. Located at the corner of Cass and Ferry, close to the campus of Wayne State University, they are near the heart of the city, its people and its culture. And the city is their life. They thrive on excitement, the fast pace. Young intellectuals, entertainers and artists are their friends.

Although they dream someday of having a second home in the country, modern suburbia does not now fit their needs. Because their rent is low (only $70 a month), they can do many things which would be impossible were they living in an expensive suburban project.

With a little ingenuity, design and splashes of paint they have transformed their half of a dark, drab fifth floor into a bright, gay home which reflects their personality, their lives and their era.

But "living at the top" of an old urban building has its disadvantages. For instance, the elevator has not been in operation since 1942. Sprinting up five flights of stairs can sometimes be quite an ordeal for friends of the couple. One acquaintance, who Chuck says is somewhat overweight, is able to survive the climb. But when he reaches the top, he is barely able to announce his arrival with one short rap on the door before collapsing on a nearby bench. "But this is a good way for them to get rid of their aggressions," Chuck says of their friends.

The steep climb no longer poses a problem for Chuck and Joni, who feel the daily exercise is good for keeping in shape. But Joni admits that on a trip to the grocery it is wise not to forget anything.

Chuck moved in three years ago, in his bachelor days, but when he and Joni were married eight months ago, things began to take shape and the apartment turned mod. "Now,", says Chuck, "it's camp."

Working together in the afternoons and between shows, they started out on what friends considered a "hopeless cause." Chuck is manager, organizer and chief construction expert, while Joni is in charge of ideas, painting and decorating.

When they first began, Chuck considered writing a book on repairing and modernizing an urban apartment. But now it seems that the job itself demands his undivided attention.

Occasionally friends come to their rescue, but mostly they do everything on their own -- from manipulating long sections of plywood up the narrow, treacherous stairway to shingling the bathroom wall to cover up the falling plaster.

They haunt estate sales and prowl through antique shops. Often when they leave work at 3 or 4 a.m., they window shop for bargains in out-of-the-way places. "The next day," Chuck says, "if we feel affluent, we go out poking around and buy some things."

These nocturnal quests have yielded such goodies as a black bear rug (for only $5) which now sprawls in front of a couch they bought for $15. They bartered for the couch and Joni found this much more exciting than just straight buying.

They like to experiment for special effect. Trunks intrigue them. They boast three such accessories. Two were gifts and one they found abandoned in an alley. One claims both gift and heirloom status. It originally belonged to Chuck's grandfather, who used it when he was in college.

Another item which rates high on their "special effect" list is a set of stained glass windows depicting a pastoral scene in brilliant blues, reds and greens. This specialty, a birthday gift for Joni, is destined to become part of their bathroom shower partition.

Joni, who once studied art, has her decorating department well in hand. Because they like country colors, she has carried the antique golds, reds and greens throughout the decor. From the glazed trunk tops to the Oriental paintings, design and planning is evident.

A whisky advertisement, antiqued and framed in red, is one of Joni's favorite objects d'art. "I hung this in protest of the rising tide of conformity," she says with a sparkle of rebellion in her large blue eyes.

Their private "urban renewal" project has meant a lot of hard work, but they've "loved every minute of it." And it hasn't been a haphazard operation. They take one room at a time and work there until they finish.

What do their friends think of this "kookie" dream? Chuck says they reveal their attitude the minute they walk in. Their reactions vary from "It's a gas!" to "you did this...and you're only renting!" But most of their guest marvel at the change. There's something different to investigate on every visit.

As for Chuck and Joni, they are optimistic. Although the building is located in a section that is destined to be torn down for urban renewal, and they don't hold a lease, they believe it will stand for at least 10 more years. "In the meantime," Chuck says, "maybe we'll get rich and buy the building."

But for the present, they concentrate on their assets and dismiss the element of insecurity. "This is the chance one has to take in renting in this area," says Chuck.