Property owners on the East Side could reap huge profits in coming years, with the large-scale public investment in the long-awaited Second Avenue subway expected to lead to a corresponding spike in real estate prices.

Although years away, when the completion of the “T-line” is eventually realized, real estate experts say, the added transit would add momentum to an already-transforming East Side.

With the large-scale development being planned at the former Con Ed site between 34th and 41st streets, a United Nations that is attempting to expand, a rezoning slated for 125th Street, and a planned esplanade for the East River waterfront, supporters of the subway line say it will boost the area’s momentum and open up new opportunities for residents who for decades have clamored for a second subway line.

The Second Avenue line has been on the drawing board since at least the 1920s, as planners have long eyed an eastern sibling for the perennially crowded Lexington Avenue line. Political and economic constraints — particularly the extraordinary cost of tunneling under the well-developed East Side — have repeatedly put a halt to any progress, and the line’s groundbreaking in April was at least the fourth since the 1970s. The cost could exceed $15 billion for the entire line, which would run between the financial district and 125th Street, though funding has been secured only for the first $4 billion segment.

Advocates of the line have touted the potential economic benefits of the investment, which would produce rewards by decreasing commute times throughout the East Side and spurring greater real estate prices and new development.

High land values, especially in subway-dependent Manhattan, traditionally follow the path of mass transit, with seniors, young children, and Wall Street commuters all benefiting from the increased ease of access.

For now, real estate experts caution that a big bump in prices along the project’s footprint is still years off, a result of the distant completion date — the final phase of construction is slated to end in 2023 — and the lack of a committed funding source for much of the line.

On the Upper East Side, where the first section of the line is scheduled for completion in 2013, brokers say the prospect of years of constant construction is driving some buyers away.

“People are wary because of its inconvenience, and people are always wary about an unknown,” an Upper East Side broker for the Corcoran Group, Wendy Sarasohn, said.

Still, others are lured by the long-term investment value, Ms. Sarasohn said, leaving prices relatively unchanged in comparison to nearby neighborhoods.

“I can’t say that I’ve seen a pattern or a surge, or any change in pricing,” a partner at the appraisal firm Miller Samuel, Jonathan Miller, said.

“In some respects, it might provide a drag on price growth in the short term — and that would be recouped and likely accelerated” upon completion, Mr. Miller said.

High property values on the Upper East Side have long been concentrated toward Central Park, as land prices gradually decline with movement eastward. Upon completion of the subway line, developers and brokers say the prices along First and Second avenues could come closer to narrowing the gap with the high-end havens of Park and Fifth avenues.

“It’s always been known that First Avenue and Second have always been at a great discount,” in large part because of the distance from a subway, the developer of a nearly completed condominium tower on East End Avenue and 88th Street, Orin Wilf, said.

With the first phase of the subway still years off, Mr. Wilf said landlords and investors see a long-term opportunity, one for which buyers of individual condos do not have the patience.

“As an investor of properties, it’s a good time to buy right now on First and Second avenues for 10 to 15 years down the road,” he said.

A 2003 study by the Regional Plan Association found that the line would open up opportunities for new development throughout the project’s footprint. The report cited the possibility to expand a health services corridor on the Upper East Side, and open up the relatively underdeveloped East Harlem area to new investment.

“That’s an area that is softer and has been bypassed by developers for a long time,” an author of the report who is a senior fellow at the Regional Plan Association, Jeffrey Zupan, said. By connecting into the growing 125th Street corridor, which includes a Metro North station, the project will open the upper sections of the line to large levels of new development, Mr. Zupan said.

The section of the line that carries one of the project’s greatest benefits — a far East Side connection to the financial district — is the last scheduled to be completed and could take more than 15 years.

Many of the stops south of Midtown connect into already existing stations, though supporters say the added transit opportunities will still spur increased real estate value throughout the line. The substantial investment required for the line has irked its critics, who claim other transit projects could see far more return on the dollar. The Upper East Side, they say, is already one of the most densely populated areas in the city.

In 2003, the Partnership for New York City issued a report weighing the benefits of numerous potential transportation projects, and found investments such as a new Pennsylvania Station and a Long Island Rail Road connection to Grand Central Terminal had a far greater payback in terms of spurring economic development.

“The Second Avenue subway is primarily a transportation project as opposed to an economic development project,” the president of the Partnership, Kathryn Wylde, said.

The Upper East Side is already well developed, Ms. Wylde said, and other projects such as a connector from Lower Manhattan to John F. Kennedy International Airport would spur construction in areas that investors are traditionally less appealing to investors.

“The benefits there multiply with downtown Brooklyn and Jamaica,” Ms. Wylde said of the JFK connector project, “and those are areas that clearly have significant development opportunities that the improved transit is a catalyst for creating.”