As the implications of the creation of a “historic district” in the Fieldston area in Riverdale become clearer, a growing number of residents are joining a last-ditch effort to thwart the plan – and they are putting their own money on the line.

Approval by the City Council is all that stands in the way of 257 properties in the elite private community from being designated as historic landmarks. If this happens, homeowners would have to win approval from the city’s Landmarks Preservation Commission to make any exterior changes to their homes. Because the process is well under way, homeowners already must get such approval.

A new group has been formed to fight the plan, the Fieldston Homeowners Association.

Although a majority of the homeowners now oppose landmarking, the city could move ahead even if all of the residents opposed the plan. In their effort to prevail with the City Council, the Fieldston Homeowners Association has hired a prominent lobbying firm, Davidoff, Malito & Hutcher, and a politically savvy public relations company, George Arzt Communications.

In a letter delivered to all homes in the community, the group asserts: “No one disagrees that our community is special, but special – with 76 different architects and 23 different architectural styles – does not make us historic.”

It added: “Future generations will be able to live in this special place without landmarking, just as they have done for more than 70 years.”

In a second letter asking families to join those who have already contributed $2,000 to finance the effort, homeowner Lewis Weinger warns that the choice is grim.

“Contribute now to avoid paying later to landmark commission architects, arborists, lawyers, and expediters and/or by having lower sales prices,” the letter says.

Mr. Weinger also warns that homeowners run the risk of being caught up in government bureaucracy, noting that if the landmarking effort is derailed there will be “significant savings in time, money, and aggravation (beside the shocking possibility that our renovation plans would not be approved as we would like).”

In fact, the association asserts that homeowners are already being caught in the web of regulation. “Kathy and Dan Roger … have spent the last several months and much money trying to get their sprinkler and alarm system plans approved by LPC, so far without success.”

None of this is dissuading the plan’s leading backer, Council Member Oliver Koppell, who said the council usually defers to the judgment of the local council member and that there is no reason to think that this won’t be the case now. Mr. Koppell is a Fieldston homeowner himself, but has come under fire from opponents for listing his own home for sale with his wife’s real estate brokerage.

“This was done for business reasons,” Mr. Koppell said. “We listed this to show a prestige property among those offered by my wife’s brokerage.” The home, which has been listed for a year and a half with a $2.75 million price tag, hasn’t been sold.

“This will pass the council,” Mr. Koppell said, “and homeowners will see that the value of their properties will be enhanced.”

The matter is due for a public hearing by the council at 11 a.m. on March 28 at City Hall.