The definition of a loft apartment can vary, depending on the city/region that you are in. However, over time, the usage seems to be convening towards one meaning over the other. We'll explain the differences between the two use cases.
1.) MOST COMMON USAGE OF THE TERM "LOFT APARTMENT" TODAY
A loft apartment is generally defined as a large, open space, usually without any internal walls (except for the bathroom!), and, up until recently, usually in one-time commercial or industrial buildings that have been converted into residential apartments. In addition to their wide-open feel, loft apartments are also characterized by high ceilings; exposed piping, ventilator tubes, and support beams and poles; wooden or concrete floors; as well as oversized, and often floor-to-ceiling, windows.
Loft apartments of this sort can be found in many neighborhoods and cities these days, although they are most often associated with NYC, where loft living is commonly assumed to have been born. Certain areas of Manhattan and Brooklyn, however, are most likely to have loft apartments from the original wave of commercial-to-residential rezoning, including Soho, Tribeca, the Meatpacking District, Bushwick, Williamsburg, and several other pockets of the city that were commercial and/or the home of light industry well into the post-war years.
Loft apartments from this era of first-, second-, and third-wave conversions are sometimes referred to as "hard lofts" by real estate people, which simply means that these loft apartments are located within buildings that were once used for something else. "Soft lofts" are loft apartments which were constructed solely as residences, often in the last two decades, to take advantage of both the look and feel of loft-living, as well as the popularity of loft apartments. Often, these apartments are described as "loft-like" rather than as true lofts.
There is some disagreement about the location of New York City's first round of "hard lofts", but most people agree that the origin of the modern conception of the loft apartment began in Soho, in the late 1960s and early 1970s. At the time Soho—the historic and architecturally united area bordered by Houston, West Broadway, Canal, and Lafayette, running north to east--was largely deserted, having just ended it's time as "an industrial wasteland" of sweat shops and the like (and before THAT, the neighborhood was home to more bars and brothels than anywhere else in town… but that's another story).
Artists and craftspeople, usually broke, creatively driven and seeking space in which to both work and (often secretly and illegally) live, began setting up their studios in these old factories and warehouses. As the neighborhood became more populated, outsiders saw how amazing some of the loft apartments were (or, more likely, they imagined what professional architects and designers could do with all that space), and a NYC love affair was born.
Decorating NYC loft apartments, with all of that uninterrupted square footage, has its unique challenges and rewards. Some owners and renters of loft apartments prefer to keep the space minimal and open: a huge beautiful bed here, a mammoth dining table there, industrial racks and shelving doing the duty of dressers and closets, all out in the open with little more than rugs or other flooring delineating the different areas of the home. Other people, especially families who live in loft apartments, put up temporary screens or rolling walls in order to achieve an acceptable level of privacy when needed, without losing that great open feeling that make loft living so special.
Families living in lofts generally opt to put up walls separating the bedrooms, for reasons of privacy. However, the rest of the apartment is wide open, and due to the large size (generally 4000-10,000+ sq ft), that still leaves a large amount of open living space.
2.) SECONDARY MEANING OF "LOFT APARTMENT"
In some areas of the county, a loft apartment refers to an apartment that has an upstairs/overhead loft area that is partially open to, and overlooks, the main living space. Usually this mezzanine is a bedroom, although in certain circumstance it could be a home office space, a sitting room, etc. In certain regions, these are also referred to as "California loft apartments," a term dating back to the 1970's.
Originally these loft spaces used railings made of wood or iron to keep people from falling from the upstairs level. While many people enjoy the open feel that provides, a more modern trend is to simply put up a half-wall. This provide a greater level of privacy, especially if the area overlooking the main living area is used as a bedroom. At the least, it provides "cover" if you haven't made your bed when guests arrive!
Sometimes this style of apartment is a true duplex apartment. In other instances, it may just be in a unit with extra high ceilings, and the loft/mezzanine is elevated or one of the main floor rooms. Often times the upstairs area doesn't have normal ceiling heights. Clearance can range from about 4 foot for sleeping lofts (intended for your mattress to be on the floor), to full standard 9' ceilings.
Access is usually via a normal staircase, or spiral staircase, depending on the apartment layout. In some cases, such as a duplex, the upstairs may also have it's own separate entrance. In rare cases (primarily when the space is mainly a sleep loft mean soley for mattresses and short furniture) access may be via a permanent ladder.
However, these days, as industrial areas in many cities have been reborn as trendy residential and restaurant/bar filled areas, the definition of a loft apartment as being a large, open, industrial feeling apartment in a converted building has taken hold. Apartments with a loft overlooking the main living area are now described as such. You'll see ads that say "loft bedroom" or "1 bedroom with loft sleeping area" as not uncommon ways of describing these units. In these instances, the loft becomes a feature within the apartment, not a way of describing the apartment itself.
"What Is A Loft Apartment?" Urban Edge. N.d. Web. https://urbanedge.apartments/loft-apartment-definition/