What Do I Call This Thing?

A Brief History of ADU Terms and Names

One of the most common debates with this building type is the simple question of what to call it. ADUs go by a myriad names, but what you call them has a lot to do with where you grew up and the housing types that were typical for your region.

ADUs are a building type that predates modern zoning, when function outdrove most other considerations. Let's take a look at some of the most common synonyms for ADUs and even a few outliers that are just interesting.

The Foundation: Terms that Predate Zoning

Dower House

Before there was a Granny Flat there was a Dower House. On English, Scottish, Welsh, and Irish estates, this was a secondary house on the grounds where the widow of the previous owner lived. She was the "dowager," and she moved out of the main house when her husband died and the heir took over, or when the heir married.

The Dower House is perhaps the most direct ancestor of the modern ADU: a smaller dwelling on the same land, occupied by an aging family member, built specifically to allow aging in place without having to leave the estate.

Carriage House and Coach House

These two terms mean the same thing — it just depends on whether you live in The United States or Great Britain. Carriage House originated in America and is the more modern term updated from the original British Coach House, a term for the same structure.

Carriage Houses have the distinction of being the predecessors to the modern dwelling unit above a vehicle carport or garage. Before the advent of the automobile, Carriage Houses served to provide a storage place for horses, carriages, and tack. To make it officially a Carriage House, there were living quarters on the second floor to provide the coachman or groom a place to stay. A direct line can be traced from classic Carriage Houses to modern Garage Apartments.

Terms of Endearment: The Family Units

Granny Flat

While a relatively common term in the US, the name Granny Flat can also be found in Great Britain, Australia, and New Zealand — all describing a similar housing type. Granny Flats, as the name implies, began as a secondary dwelling designed specifically for an aging family member, typically a matriarchal figure.

This term has generally been removed from the codes due to Granny Flats being, well, not for just grannies anymore.

Mother-in-Law Suite, In-Law Unit, In-Law Apartment

Another set of terms that are very descriptive in their titles. While typically American in origin and usage, there is no clear origin for these terms. This points to these terms remaining in the vernacular rather than making their way into official codes or definitions.

These units often were not detached but rather a portion or extension of the main house that would accommodate multiple generations living on the same property. These terms don't find their way into most planning texts because they describe the end-user, not the building type.

Ohana Unit

Ohana means Family in Hawaiian. While less prescriptive than the In-Law variations, Ohana Units are intended to fulfill the same function — all for aging in place. Ohana Units were first introduced in Hawaii in 1981 and remain a staple building type in Honolulu. Hawaii has since created a distinction between ADUs and Ohana Units, but with origins spanning nearly a half century, Hawaii was ahead of the curve with their wonderful Ohana Units.

Casita

Meaning "little house" in Spanish, Casita is a term one might hear in places like Texas, Arizona, New Mexico and even southern California. Casitas have long stood as a desirable building type for residents of the southwestern region and while the term has never really gained traction in legal circles, Casitas remain a symbol of a more romantic time.

The Bureaucratic Terms

Second Unit, Secondary Unit, Secondary Suite, Secondary Apartment

This arrangement of terms shares a similar theme: each is trying to tell the audience that this structure is not the main house. This was the preferred set of descriptors for these units in the early days of planning for such structures, but they have generally been left behind for the more acronym-ready Accessory Dwelling Unit, or ADU.

Accessory Apartment and ECHO Housing

For much of the early history of ADUs, Accessory Apartment was the preferred term for this unit type. As this housing type has evolved over time, the term Apartment has been removed in favor of the broader term Dwelling. Accessory Apartments in Austin used to refer to a dwelling within the main house that was limited in size and scope to being only able to be occupied by a family member.

ECHO Housing (Elder Cottage Housing Opportunity) was an American housing initiative that piggy-backed off Australia's Movable Units Scheme, which provided residents who qualified access to mobile housing units that could be removed and repurposed when no longer needed.

Accessory Dwelling Unit (ADU)

This term became the preferred and most common of the modern terms due mostly to it being written into the most planning code. Accessory Dwelling Unit and eventually the common acronym ADU have become the standard simply by default — it was the most used term, so it became the standard. Too bad that isn't more interesting.

Junior ADU (JADU)

This is a California-specific housing type that is similar to Austin's old Accessory Apartments. JADUs must be contained within the main house and are limited in size. Austin's equivalent has been eliminated.

Laneway House

A purely Canadian term — Vancouver to be exact — a Laneway House is a secondary unit that faces a rear lane, or alley in America. One might not encounter this term in the wild, but we included it to give the reader a well-rounded education on the topic.

One of these things is not like the other

These are terms that are used for ADU but are in fact not the same at all.

Guest House

While colloquially Guest House might seem like the perfect term, it has a distinctly different connotation from an ADU. In code definition terms, Guest Houses typically have a kitchen or a bathroom, but not both. A Guest House might more resemble a hotel room rather than a house or apartment.

Tiny House

A Tiny House is the most mistaken housing type we get. There are specific codes that deal with Tiny Houses separately from ADUs. While most jurisdictions define a Tiny House as being 400 square feet or less, the main difference is the foundation type (or more typically, the lack of one). Tiny Houses are intended to be mobile or semi-permanent while ADUs are intended to be permanent. Those are the main differences between the two.

What does it all mean? Or why this is all moot in Austin

In 2023 Austin removed all references to accessory uses in the Land Development Code. Instead, the distinction is simply dwelling or not dwelling. And what makes it a dwelling? In short, the plumbing. That's basically it. If it has habitable space, a full bathroom, and a sink outside of that bathroom, it is a dwelling. Remove one of those functions and it is not.

So what should you call an ADU? Call it whatever you want — just make sure you call us first.