The marshy land remained barren until the 1920's, when Realty Associates bought the property and dredged streams, using the sand for fill.

The Lilliputian colony was so successful that many residents decided to stay year round, and Realty Associates proceeded to develop the remainder of the area during the next decade.

And in 1986, the lot's owner, 28th-29th Realty Associates, filed plans for a 31-story apartment house on the site.

HFS Inc. said yesterday it had agreed to buy Electronic Realty Associates, the world's fourth-largest franchiser of residential real estate brokerages, for about $46 million.

ERA Real Estate was founded by Jim Jackson in 1972 as Electronic Realty Associates, Inc..

Last week, Electronic Realty Associates of Kansas City, Mo., unveiled a worldwide computerized database of its listings.

In 1920, Realty Associates, a speculative real-estate builder, began constructing a middle-class summer resort there.

Electronic Realty Associates (ERA), the national franchise, says that two studies showed that homes carrying such protection sell faster and for slightly more money than those without it.

The area, named for Wolfert Gerritsen, a 17th-century settler, was mostly marshland until around 1920, when a company called Realty Associates began building summer homes.

His parents own American Realty Associates, a commercial real estate and house-building business in Atlanta.