About the same time that Hardenbergh was working with Edward S. Clark of the Singer Sewing Machine Company, designing Kingfisher Tower, rowhouses the Dakota Apartments and other buildings, he was also working for another Singer executive on an all-but-forgotten project upstate. In Glen Spey, a hamlet in the Town of Lumberland, Sullivan County, N.Y., he designed a mansion known as Loch Ada, or Allsworth, for William Ross Proctor, the Singer Company's treasurer.

Loch Ada was unoccupied after about the 1920s, according to Lumberland Town Historian, Frank V. Schwarz, and it burned down in 1996. However, there may be additional works by Hardenbergh that still stand in Glen Spey. Loch Ada's gatehouse still stands as a private residence. And there were four additional mansions (and associated outbuildings) built in Glen Spey during the eighteen nineties and nineteen aughts for children of George Ross Mackenzie, who was president of the Singer Company from 1882-1889. These four mansions are:

• Burn Brae, built for Margaret Ross Mackenzie Elkin. This stands today fully-restored as a training resort for competitive cyclists run by Mike Fraysse at 573 High Road.
• Ardmore, built for Rebecca Mackenzie Vanderbeek. This has been enlarged, and is known today as Verkhovyna-Mountain View Resort, a Ukrainian-American cultural center, at 369 High Road.
• Wold Croft, built for James Stone Mackenzie. Today, this building serves as Camp Chayei Sura, a Hasidic summer resort at 505 High Road.
• Bramble Brae, built for Grace Mackenzie Ewing. This stood adjacent to Ardmore, but was demolished in 1975. Several of Burn Brae's outbuildings remain standing in a dilapidated condition.

As a caveat, there is no documentation linking Hardenbergh to any of the buildings in Glen Spey except the Loch Ada mansion. Evidence that he may have designed them is circumstantial, and four-fold. First, they were built during the period of Hardenbergh's peak activity. Second, they were built in a town where Hardenbergh had been active. Third, they were commissioned by relatives of an executive at the Singer Sewing Machine Company, George Ross Mackenzie. (Mackenzie's predecessor as president, Edward S. Clark, built the Dakota, Kingfisher Tower, the Van Corlear, the Ontiora, the Wyoming, and two sets of rowhouses on West 73rd Street. Mackenzie's colleague, treasurer William Ross Proctor, built Loch Ada.) Fourth, the buildings appear to be of a quality and style in line with Hardenbergh's known works.

Many thanks to Frank V. Schwarz, Lumberland Town Historian, for a thorough tour of Glen Spey.

The front, in better condition. (Photo courtesy of Frank V. Schwarz, Lumberland Town Historian.)

A later photograph of the front. (Photo courtesy of Frank V. Schwarz, Lumberland Town Historian.)

A close-up of the rear of the building. The chimney has an interesting detail between the first and second floors. (Photo courtesy of Frank V. Schwarz, Lumberland Town Historian.)

Upon closer inspection: Name, date and architect. (Photo courtesy of Frank V. Schwarz, Lumberland Town Historian.)

Loch Ada's gatehouse still stands as a bucolic private residence. (Photo by Bob Sheridan.)

The south facade, facing a garden. (Photo by Bob Sheridan.)

The rear, or west facade, faces a garden as well.

Burn Brae, impeccably preserved today as Mike Fraysse's training resort for competitive cyclists. (Photo by Bob Sheridan.)

Ardmore, enlarged today as the Verkhovyna Ukrainian-American Cultural Center. (Photo by Bob Sheridan.)

Wold Croft. Today, Camp Chayei Sura. (Photo by Bob Sheridan.)

Bramble Brae. (Photo courtesy of Frank V. Schwarz, Lumberland Town Historian.)

Bramble Brae's garage. (Photo by Bob Sheridan.)

A tower on the Bramble Brae property. (Photo by Bob Sheridan.)

Ardmore's gatehouse. (Photo by Bob Sheridan.)

Former post office building, now a private residence. This was built for Edward Easten Mackenzie's son, Edward Hemmond Mackenzie. (Photo by Bob Sheridan.)