Because of the length and difficulty of Windmuller's commute between his office at 20 Reade Street and his home at Woodside, he also kept an apartment in Manhattan at 19 West 46th Street. It's probable he took this apartment soon after the building was constructed in 1865. It's still standing and is a remarkable structure, as you can see from these photos.
The photos come from a blog post — The Skinniest Building in Midtown — which gives details about the building. A reader comment says "The suggestion from [another commenter] that there might have been a twin to 19 West 46th Street is correct. Number 17 West 46th Street was its sibling, both built on a single 25-foot lot. Both were originally about 60 feet deep on the 100-foot lot, but the now-razed house at number 17 had a two-story extension that brought the back of the lower floors of the house about 15 feet closer to the rear lot line." Another says, "i used to live in this building. the top floor is its own apartment, as well as the floor below it where i used to live. theyre both two bedroom apts.
This photo shows the upper two floors. Notice the handsome decoration on the building to the left.
A third comment tells us that the large buildings on either side of no. 19 were built in the 20th century: "#21 W. 46th was built in 1930, and #15 was built in 1920."
Fire insurance maps show that nos. 17 and 19 were late additions on the block. They show an empty lot in a set of maps published in 1867, presumably from data collected in the previous couple of years.
An article in the New York Times back in 2006 gives some information on the development of the area.
In the 1860’s, the West 40’s just off Fifth were filling up with commodious private houses, almost all high-stoop brownstones, as the center of fashionable New York gradually migrated north from Murray Hill. Some of the houses were 20 or even 25 feet wide, but others were not so grand, like 19 West 46th Street, a sliver of a thing only 12½ feet across.----------
By 1910, this section of West 46th was evolving into a block of distinctive shops and stores. In 1914, The New York Times said that retailers considered it “a little superior in exclusive trade facilities and artistic building treatment” to other streets, and reported that 46th was sometimes called “the Bond Street of America.”
-- Streetscapes | West 46th Street A Block That Looks as It Did About 1930
Here are two Google street view images of the building and its neighbors.
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This birds-eye view shows the building in 1879, overshadowed by older and larger neighbors.
The city of New York. Will L. Taylor, chief draughtsman.
This detail, from an atlas of 1857-62, shows that nos. 17 and 19 were an empty lot at the time.
The is the full sheet from which the detail comes.
Maps of the city of New-York, by William Perris (New York, Perris & Browne, 1857-62); source: Early Real Estate Atlases of New York in the NYPL Digital Gallery
In this detail from a map, published in 1867, the two addresses are still a vacant lot; presumably the survey had been conducted a year or two before then because I'm pretty sure that Louis Windmuller had rented an apartment at 19 W. 46th by that time.
This is the full sheet from which comes the detail.
This detail comes from a map published in 1885.
Here's the full sheet.
Here is part of the key to the map.
In 1884, the Windmullers invited friends to their apartment at 19 W. 46th to help them celebrate their 25th wedding anniversary, as we learn from this short report in the New York Times.
The Skinniest Building in Midtown
Interview with the Scout on Gothamist
Fire Insurance, Topographic, Zoning and Property Maps of New York City
"Early Real Estate Atlases of New York"
 Click the Family History label in right-hand panel of this blog to see other posts on Louis Windmuller and his family. This extract from his entry in a biographic dictionary gives one view of his first dozen years in New York. It's misleading to say he had no acquaintances to assist him since he had the names of relatives who'd emigrated in the preceding couple of decades and one of them, his cousin Henry Lefman, became his mentor, business partner, and eventually father-in-law.
WINDMUELLER, Louis, merchant and importer, was born at Muenster, Westphalia, about 1836. After studying for a while at the Catholic college of Muenster, pecuniary difficulties compelled him to leave before graduating, and he resolved to emigrate to America. In 1853, therefore, he came to New York, landing in that city without money and with no acquaintances to assist him in finding means of support. He had an iron will, fortunately, and went to work courageously and with so much success, that by the year 1858 he had an established business of his own. In the year 1865 he formed a partnership with his old friend and countryman, Alfred Roelker, under the title of Windmueller & Hoelker, which firm now (1893) takes rank among the most prominent importing and commission houses in the country. This fire insurance map shows 15 Stone Street in 1857. It was a business establishment with no dwelling units.
-- The National cyclopaedia of American biography, Being the History of the United States as Illustrated in the Lives of the Founders, Builders, and Defenders of the Republic, and of the Men and Women who are Doing the Work and Moulding the Thought of the Present Time, Vol. 4, (J.T. White, 1895).
 Although this photo was taken in 1938, the buildings at left are similar to, if not the same as, ones that lined the street in the 1850s.
 This print shows the character of Hudson Street in 1860.
This detail from a panoramic map of 1881 shows 222 Bloomfield. You can view the whole of this excellent map here.
 This modern photo of Dean Street (which I think includes 281) shows houses that may date back to the 1860s.
Here's an 1869 map of the block.
 I hope to devote another blog post to Windmuller's Woodside estate.
 This is an interesting blog by a location hunter who says: "I work as a film location scout in New York City. My day is spent combing the streets for interesting and unique locations for feature films. In my travels, I often stumble across some pretty incredible sights, most of which go ignored daily by thousands of New Yorkers in too much of a rush to pay attention. As it happens, it's my job to pay attention, and I've started this blog to keep a record of what I see."
Here's an update: "Almost a year ago, I wrote a post about my favorite building in midtown, this chopped-in-half brownstone at 19 W 46th Street. The other day, I got a note that a Lego fan had built a Lego version of the building based on my pictures!" -- LEGOizing NY
 Yet another commenter says "The city used to sell lots in a standard 25 foot width. Some houses were built to this size, but some developers would buy four lots and build five houses (for 20' each), or buy three lots and put up four houses (for 18.75' each), or various other combinations that lead to the standard widths of Manhattan townhouses to this day. The most extreme measure was to buy a single lot and put up two houses, which would result in two 12.5' houses, side by side. I believe this was rarely done in speculative development, but it would sometimes happen that a father would buy a lot and split it between two children, for example."
 According to the New York Times, 19 W. 46th was constructed in 1865.