The New York City Landmarks Preservation Commission is poised today to designate a privately owned community in the Riverdale section of the Bronx, Fieldston, as a historic district. Although the designation would need to be approved by the City Planning Commission, the City Council, and Mayor Bloomberg, it would immediately be implemented.

In effect, the move would make all of the houses in the development, except those of recent vintage, landmarks. Such a designation prevents owners from making exterior changes to their homes, major or minor, without approval of the Landmarks panel.

In recent weeks the issue has taken on a new dimension, as many among the area’s growing Orthodox Jewish population have voiced objections to the plan, suggesting that they are the targets of an effort to slow the growth of their numbers in the community. Homes in Fieldston, many of which were built in the 1920s, typically sell for prices in excess of $1 million. But these homes often need extensive renovation, particularly for larger Orthodox families.

Opponents of the plan noted that two previous landmarking efforts in Riverdale also seemed to focus on the Orthodox community. In 1996, a building purchased as a dormitory for the Telshe Yeshiva was designated as a landmark, upsetting the school’s expansion plans. Two years ago, the renovation of a house purchased by an Orthodox family was halted after a petition to landmark the structure was filed with the commission. The petition was withdrawn when it became clear that there was no clear historic record regarding the structure, but the family was forced to pay thousands of dollars to defend its right to renovate. Ultimately, the house was put back on the market and sold.

A leader of the opposition, Fieldston homeowner Alan Rosenthal, has been gathering signatures in an effort to postpone the landmark designation by 90 days. At the same time, the opponents are said to have hired a lobbyist and an attorney to fight the plan.

In 2003, a ballot of the 261 property owners showed 121 supporting the landmark designation, 102 opposed, and 38 abstaining or not voting. Mr. Rosenthal claims that he now has signatures from 125 property owners opposing the plan.

“The tide is turning,” Mr. Rosenthal said. “We are bordering on the majority.” Significantly, both Manhattan College and the Horace Mann School, institutions that hold property within Fieldston, have signed the petition for delay.

This does not faze one of the proposal’s key supporters, City Council Member Oliver Koppell. “I believe that ultimately all of the residents will see how this proposal will benefit them and enhance the value of their properties.” Mr. Koppell pledges to continue his support and shepherd the plan through the council once it has been approved by the Landmarks and City Planning commissions. “This will be approved,” he vowed.