The addition, 17 stories high, preserved the color and vertically massed profile of the original, but was more traditionally ornate in its design, with grander entranceways and more neoclassical touches. The building reflected in its design the transition taking place in skyscraper design from load-bearing walls to steel frame construction . The Katahdin, built first, used the same iron framed masonry construction as the original. The Wachusett was entirely steel framed. Where the north half required great thicknesses of brick in the load-bearing walls, the addition employed only a thin facing of brick and terracotta trim, affording larger expanses of glass and faster, less expensive construction. The south half cost 15 percent less, weighed 15 percent less, and had 15 percent more rentable space than the north half. Connected on every floor except the top one and sharing a common basement, each of the four component buildings was equipped with its own heating system, elevators, stairs, and plumbing to facilitate a separate sale if required. The combined final cost in 1893 was $2.5 million ($ in dollars). When complete, the Monadnock was the largest office building in the world, with 1,200 rooms and an occupancy of over 6,000. The "Chicago Daily Tribune" commented that the population of most Illinois towns in 1896 would fit comfortably in the building. It was a postal district unto itself, with four full-time carriers delivering mail six times a day, six days a week. It was the first building in Chicago wired for electricity, and one of the first to be fire-proofed, with hollow fire clay tiles lining the structure so that the metal frame would be protected even if the facing brick were to be destroyed. The Brooks' decision to construct a building of such scale and in such an unlikely location was vindicated by the Monadnock's success—it was the most profitable investment they ever made. "The Economist," a Chicago real estate journal, conceded in 1892 that:  Early tenants, according to Rand McNally, included "great corporations, banks, and professional men ... among them the Santa Fe, the Michigan Central, and the Chicago & Alton Railroads, and the American Exchange National and Globe Savings Banks". In 1897, the Union Elevated Railroad Company opened the Union Loop line of the Chicago "L", the last leg of which ran immediately alongside the Van Buren side of the building. Aldis filed suit against the "L" in 1901 for $300,000 in damages ($ in dollars), complaining that:  Aldis lost the case, but won on appeal, when the Supreme Court of Illinois found that owners of property abutting the "L" lines could recover damages if the property had been injured by noise, vibration, or the blocking of light, paving the way for many lawsuits to follow.