Occasionally, however, co-ops choose not to heed Mr. Glass’s advice.

There was the time when a television personality bought a floor-through apartment in a prewar building and proposed converting it into a one-bedroom by changing all the additional bedrooms into bathrooms or walk-in closets. Mr. Glass worried that it would hurt resale values, but the co-op let it go, saying the next owner would just change it back at his or her expense.

Mr. Glass shrugs it off. After years of watching the wealthy flood Manhattan and do extensive renovations, no excess surprises him.

He has seen it all: the multimillion-dollar gut renovation that spread across three years. The couple who bought on Fifth Avenue facing the park and then requested so many plantings on a terrace that it blocked their views. A sudden insistence on $30,000 French stoves that require more ventilation than can be reasonably accommodated. He sighs, “Odds are no one will really cook on them anyway.”

For the record, Mr. Glass lived in a rental most of his life. Only recently did he buy into a co-op, in Union Square. But he has no plans to get involved with its decision making.

Still, perhaps no one knows the interior life of co-ops better than he.

Mr. Glass grew up in the Bronx, the son of an architect. His father, M. Milton Glass, was prominent in city planning and a partner in the architecture firm of Meyer, Whittlesey & Glass, which designed Butterfield House on 12th Street and 220 Central Park South, among others. He was always intrigued by the family business and received a five-year degree in architecture from Cornell University in 1957.

He meant to get into design, of course. But his real business, that of reviewing others’ work, developed in the 1960s. It came about almost by accident when a client called his father to resolve a dispute. He sent Elliott, and the son began being called in to write reports.

The business built slowly in the 1970s and then exploded with the boom in converting rentals into co-ops in the 1980s. Now, Mr. Glass says, even though his services run $350 an hour, he has more work then he can handle.