PERSPECTIVES: DOWNTOWN LEASING; SHUNNING THE BACK-OFFICE SPINOFF

MAJOR leases in new office buildings are a little like old-fashioned marriages.

Brokers introduce the parties and the terms of the alliance are closely negotiated. Then comes the ceremony, sometimes at City Hall.

One such ceremony occurred Dec. 18 when Mayor Koch announced that Thomson McKinnon Inc., the nation's sixth largest privately held brokerage firm, had signed a lease to take 560,000 square feet in a new building that Howard Ronson is developing downtown called Financial Square.

The lease was a notable demonstration that consolidation rather than dispersion is still the goal of many business organizations when they relocate. The separation of so-called back-office functions from front-office functions was specifically avoided in the Thomson McKinnon case.

From another standpoint the lease showed the strength of demand for large blocs of space downtown. That is an especially heartening sign in view of the level of construction of buildings with large floor sizes.

Roughly 13 million square feet of new space is estimated to be under construction in the downtown market, most easily defined as the area south of Chambers Street, for occupancy next year or in 1987. With the Thomson McKinnon lease, about four million square feet of the total remains unspoken for, the Real Board of New York estimates.

As this space gets absorbed, however, the inventory of space available for prime lease or sublease in existing buildings grows, as tenants move out and consolidate in new buildings. Brokers estimate that about 13 million square feet is available for prime lease or sublease in existing buildings.

The total downtown inventory, including buildings in construction, is estimated at about 120 million square feet. This suggests that a total of 16 million square feet is on the market, or an availability rate of 13 percent of the total supply in new and existing buildings.

''We've been pleasantly surprised by the strength of the existing buildings,'' said Arthur Margon, director of research for the Real Estate Board. He said that large companies that are moving into new buildings - including Shearson American Express and Merrill Lynch - were keeping some of space in older buildings rather than relinquishing it entirely as had been widely expected. Therefore the availability picture has not expanded as drastically as had been expected when the current wave of new construction started.

The Financial Square project consists of one million square feet of 29 column-free floors of about 34,000 square feet on a site bounded by South Street and Front Street on the east and west, and Gouverneur Lane and Old Slip on the north and south, formerly occupied by the Federal Assay building. The property is at the eastern end of the financial district, just a block south of Wall Street.

Edward Durell Stone Associates designed the building in such a way that there are two totally independent sections, with separate heating and cooling systems as well as separate entrances.

Thomson McKinnon will occupy the first 17 floors, with two entrances, one at Old Slip and Front Street and the other at Old Slip and South Street. The firm signed a 15-year lease that it said had a total value of $300 million, with Williamson Picket Gross, Inc., as the broker.

Mr. Ronson has been the principal builder in re-cent years of smaller new buildings downtown on ''infill' sites that others had passed by. He is getting still another under way at present, a 350,000-square-foot structure at 5 Whitehall.

But the Financial Square project represents a step in a different direction, toward the building with a larger floorplate, which other builders have also favored downtown.

''Companies are looking for 500,000 square feet, a million square feet and more,'' Mr. Ronson said. ''That was unheard of five years ago.''

He also says that the most rapid area of tenant growth nowadays is in front-office functions.

''Five or six years ago people talked of back-office growth,'' he said. ''Now the explosion in growth is in front-office functions. We see no difference in the 1990's between front-office and back-office functions.''

A financial company that illustrates some of these principles is Thomson McKinnon, a 100-year-old securities firm with 1,500 employees downtown. It has four floors in One New York Plaza, six floors in One State Street Plaza, and lesser space elsewhere.

When it set out a year and a half ago to search for space to accommodate a continuing expansion, it intended to hold together rather than disperse all the activities in the headquarters division, and give them expansion room within the new facility as well, according to Ida Brancato, senior executive vice president.

Nine hundred of the 1,500 employees can be described as ''back-office'' people, either in operational or administrative departments. They are called that becuase they provide support in marketing, sales, research and the processing of securities transactions, or in such administrative tasks such as accounting, personnel or payroll.

The remaining 600 downtown employees are in ''production'' areas - meaning that they produce revenue through the direct provision of client services. They include the traders, account executives, investment banking advisers and others. The back-office and front-office people interact with each other int he daily course of business.

''We would need to increase our space by 30 percent just to provide breathing space for our current working population,'' Miss Brancato said. In the new building the firm will keep all its space, giving itself room for future expansion, rather than sublease any of it, she said.

Financial services firms that hold their functions together are major users of space being created on large-floor buildings downtown. These buildings include Larry A. Silverstein's 7 World Trade Center, with 1.7 million square feet of space; George Klein's 60 Wall Street, which Morgan guaranty Trust Company has leased; and the various buildings within Olympia & York's World Financial Center, at Battery Park City.

If the space available in these and other buildings in construction downtown is added to the space still awaiting occupancy in new buildings that have already opened, the total inventory of as-yet-nspoken-for new space downtown amounts to about 19 million square feet, specialists calculate.

By comparison, the total office inventory in downtown Philadelphia amounts to about 20 million square feet. Boston has aobut 22 million square feet, as does Los Angeles.

Figures on downtown leasing indicate that 7.7 million square feet was taken this year, of which 4.1 million square feet was in new buildings. Developers regard this as a favorable omen for the absorption of new space under construction within one to two years.

LEASES for the larger tenants will have to be written so that over a 10-year period they produce a rent averaging at least $40 a square foot, according to brokers. If debt service on a $250-a-square-foot development project comes to about $25 a square foot a year, and operating costs and taxes total about $12 a square foot, $40 a square foot per year would be close to a break-even figure.

Actual lease terms are more complicaTed, since much depends on how much interior work the developer is providing, or how large an allowance is being granted for it. Brokers say they calculate that many tenants will need $70 to $90 a square foot in interior improvements for the tenant.

In the downtown market space is available in a wide price range, brokers say, ranging upward from about $17 a square foot. Tower floor in prewar Wall Street buildings probably would bring rents of about $30 a square foot.

The influx of foreign bank, financial companies and trading firms has bolstered the market greatly in recent years, but Mr. Ronson, who is British, believes that there is more to come.

''There are still major banks and corporations and investment firms overseas that don't have a United States presence,'' he said.