Work site muck clinging to his shoes, George Field Jr. climbs back into his pale yellow Porsche and muddies its fine interior. He could hardly be happier, though. After a recession-induced slowdown in construction, he is building again.

On four and a half acres in Warren, Mich., a bulldozer is breaking the winter ground for 436 self-storage garages. A couple of towns away, in Fraser, Field's company is roofing a total of 75,000 square feet of storage space, adding to three successful sites nearby. By May 1, the president of Your Attic Inc., headquartered in Troy, Mich., expects construction at seven more sites in Michigan.

Good statistics are hard to come by for this 20-year-old industry, but informed opinion pegs the number of ministorage locations nationwide at about 5,500; annual gross revenues are more than $1 billion. While Your Attic, with revenues in 1984 of $1.1 million, is not self-storage's biggest gun, Field, 39, the industry's lone franchisor, is generally regarded as an innovator who has shrewdly differentiated his product from his look-alike, sound-alike competition. And, where others have seen self-storage as "land-banking" -- cheaply built, revenue-generating real estate easily sacrificed to rising land values -- field saw a service industry emerging.

A decade ago, while he was crisscrossing the Southwest to get a look at the still-toddling industry, Field saw one primitive operation after another: Bob's Self-Storage, U-Lock-It. Many had no security fences. Most conducted business in a front room of the resident-manager's apartment, typically an extension of one of the storage sheds. "Managers would come out in their undershirts," Field recalls. "You could smell the cabbage cooking."

Field saw a drab industry in need of a makeover, and, with his eye on franchising popssibilities, came up with the idea of a farmhouse design to distinguish Your Attic from all the other mom-and-pop operations. Today, just outside the secured perimeter of each of his storage facilities, he builds a homey-looking, two-story farmhouse. Its ground floor provides office space; its second story, living quarters for the resident manager. And it is more easily seen by passing motorists than any sign his competitors could put up. As a corporate symbol, the farmhouse stands out in the Yellow Pages, which helps keep the telephones ringing.

Field was among the first to install computerized security gates and to offer a wide range of services and products -- rental trucks, cardboard boxes, padlocks, even inexpensive flashlights. And Field set his rental rates about 20% above his competition. He also set out after a different sort of customer. Despite the industry wisdom that pegged users of self-storage as transient apartment dwellers or condominium owners, Field claims that commercial tenants account for about 75% of his business, a figure that stands the industry average on its head.

Field requires each of his managers to join the local Chamber of Commerce and to pitch everybody in sight. Stores that pay $24 a square foot for space in nearby shopping malls, for example, can store their excess inventory at Your Attic for one-quarter to one-third that amount. Banks, medical practices, and law offices are also targeted -- Your Attic's premium prices, about $10 a square foot a year, are still below the going rates for office space. "Electronics be damned," Field says, "businesses are still generating a lot of paper."

At last count, Field had sold 29 licenses (at $40,000 each), 18 of which are being developed in partnership. Eventually, he projects some 300 to 400 Your Attic facilities nationwide, each with annual gross revenues exceeding $350,000 (2% of which will return to Your Attic, in which Field owns roughly one-third interest). "I personally expect to be making several hundred thousand dollars a year just in rental income from my properties," says Field. On his shoes there may be mud, but on his tie, gold on blue, are emblazoned dollar signs. Lots of dollar signs.