When it was erected by Horizen Global, which is headed by Michael Yanko, the 10-story building at 423 West Street facing the Hudson River was planned to contain 8 condominium apartments.

Designed by Patrick Han, the very attractive building, which has been known as Hudson Blue, was notable for the thin diagonal pier on its mostly glass facade. The building, which is 23.5 feet wide and between Perry and West 11th Streets, has a setback at the ninth floor. It is just to the north of the three modern mid-rise buildings designed by Richard Meier for two different developers.

It was planned to have a part-time attending lobby with limestone floors and a fireplace and all apartments would have fireplaces and balconies or terraces and the building has private storage areas and a wine cellar. Prices initially for 1,245-sq. ft.-units with 1.5 baths were to start at about $1,990,000.

Times have changed and in December the building was listed with just six units.

Now, however, the developer is offering the building as a single-family home. The property is now listed as a 15,000-square-foot, 21-room house with 11 bedrooms, 9 bathrooms and five-and-a-half half-bathrooms and is listed by Mara Flash Blum of Sotheby's for $21 million.

She told CityRealty.com that the change in strategy is "because we have substantial interest in the whole building over individual unit."

At $21 million, it might be considered a bargain because of its river views and the fact that another townhouse now under construction at 2 North Moore Street, several blocks south in TriBeCa has only six floors with a total of about $11,300 square feet and a $35 million pricetag. It has 5 bedrooms, 7 bathrooms, 4 powder rooms, a garage and a 47-foot, heated lap pool.

There are other choices.

The 45-foot-wide, 22,000-square-foot mansion at 22 East 72nd Street that has been occupied for several years by the Salander-O'Reilly Galleries, has been placed on the market with a $75 million pricetag.

It has grand interior spaces and was designed in 1922 by C. P. H. Gilbert for Julius Forstmann, a wool merchant. It was acquired a few years ago by Aby Rosen and Michael Fuchs for about $15,650,000.