Much of the excitement about accessory dwelling units right now stems from the recent passage of new laws in California and Oregon that make it easier for property owners to build additional living spaces on their land. Areas that were once zoned only for single-family homes are now open to the construction of new, separate accessory dwelling units, or ADUs, or adding junior accessory dwelling units (JADUs) by converting an existing bedroom or a garage into a distinct living space.
There’s a bit of confusion out there though, about how different traditional types of home construction fit into this new arrangement of things. In particular, many commentators are using the word “duplex” to describe some of these kinds of conversions. But are they really the same thing?
What is the Difference Between an ADU and a Duplex?
The difference between an ADU and a duplex is in the original design of the home that was built on the property. Accessory dwelling units are additions to a property. Duplexes are homes that were originally designed to accommodate two separate households in one building, and which have all of their own separate amenities.
Duplexes come in many different shapes, sizes and arrangements. Some duplexes are designed to be a single building that straddles a property line. In other words, you have two separate households, each living on their own property, but the buildings are actually a single structure with a shared wall dividing them down the middle.
Other duplexes are designed to work as a single property that is simply shared by two households. In many cases, properties of this type are owned by a landlord and rented out to another family or two families.
Some duplexes can be designed more similarly to apartment buildings, wherein one household occupies the lower floor of the house, and another occupies the upper floor, each with separate entrances.
This matter of separate entrances is also a critical component of a duplex. If two separate households have to enter their residence through the same entrance, it functions more like a boarding house than a duplex. The idea behind duplexes is a balance between structural efficiency and household privacy.
By contrast, accessory dwelling units are structures that are additions or complements to an existing single-family home. They are most commonly built on properties that have a reasonable amount of backyard space, or a garage that isn’t routinely used for cars or storage.
Accessory dwelling units are also always smaller than the original home on the property, whether it’s a backyard unit or a garage conversion. By contrast, duplexes are usually going to provide the same amount of square footage for both households.
It would be really easy to mix up duplexes and junior accessory dwelling units, since they do share the same building, but the key difference is that the JADUs are always smaller, and weren’t originally designed for the purpose of being a separate residential unit.
What is a Duplex?
As noted above, the very definition of a duplex is a single residential structure that houses two separate households who each use two separate entrances to access their living space.
There are some minor exceptions to that rule, such as duplex homes that are disguised as single-family homes to avoid upsetting other neighborhood residents. In those cases, though, a single front entrance merely opens to a shared hallway where two other entrances are immediately accessible. Everything else remains separate.
Duplexes have been around for a long time. They first started becoming very popular about a century ago, especially in Los Angeles, where suburban development was just barely beginning to take off at the rapid pace it became famous for.
There was always a need for higher density housing, but the suburban, detached, single-family character of many other Los Angeles developments made it important for developers to fit in with that existing style, and the expectations that went with it.
Noticing that many suburban residential properties featured detached houses with small spaces on either side of them, developers simply noticed that you could squish two of those houses together and still leave plenty of accessible space around them.
Considering how many of the detached single-family homes have side walls with few or no windows, it almost doesn’t even make a difference, except that it provides a significant savings in construction costs.
The overall effect is that it allows developers to fit a denser arrangement of homes into a neighborhood without compromising its low density appearance and character. Duplexes really are an elegant solution to a social problem.
Laws Regarding ADUs and Guest Duplexes
As with all building construction in urban areas in the developed world, duplexes and accessory dwelling units are subject to laws that can include planning ordinances at the local level, and state mandates as well.
That’s certainly the case in California, which is now implementing new laws that override previous local restrictions on the number of units that can be built on a single-family residential property.
Before 2020, many local governments in California had laws and ordinances on the books that sharply restricted the potential density of housing in certain neighborhoods, or across their entire municipality.
There are many reasons for these rules, some of which are aesthetic, and some of which have to do with actual principles of urban planning, such as traffic concerns, and environmental considerations. Some of them, though, are specifically meant to be exclusionary of lower income groups.
One of the consequences of these restrictive policies has been a sharp shortage in the supply of housing throughout California and other areas of the urbanized western United States.
The western US is a lot more rugged, geographically, then the eastern half of the country. Cities are far more sharply constrained in their growth by natural landforms such as mountains, and deserts. There’s also the problem of higher infrastructure costs, especially in the desert southwest, where cities often have to pipe their water in from hundreds of miles away.
In the East and Midwest, many cities are able to simply keep growing and expanding and spreading all the way to the horizon with no major geographical barriers.
On the other hand, major western cities such as Seattle, Portland, San Francisco, and Los Angeles have been highly dynamic centers of growth during the information age. These metropolitan areas have been growing by the millions for decades, but they’re finally starting to bump up against severe geographical barriers.
That means further growth has to come in the form of denser housing in existing built-up areas. Sometimes it makes sense to build a residential skyscraper, but those are always highly expensive and sometimes risky real estate ventures. They also tend to cater to the wealthiest members of society, who can usually afford to get the kind of house they want, where they want it, anyway.
The real problem is finding ways to provide housing for lower income members of society who need to be able to live close to where they work, because they can’t afford vehicles as easily, nor the gasoline required to commute from much further away where housing is cheaper.
Decades of work building up zoning laws that favored detached single-family suburbs made it almost impossible for adequate supplies of that kind of high density, low cost housing to be built.
And that, in a nutshell, is why the states of California and Oregon have now overridden those old restrictions protecting exclusive single-family zoning. It’s likely that many other Western states will follow suit in coming years.
These innovations may also be applicable in some ways to other regions throughout the country, such as in rugged New England, but cities there tend to be older and more densely and traditionally developed, anyway.
In any case, we’ll take a slightly closer look, too, at some of the specific laws that are now in place in the four most dynamic West Coast cities.
Los Angeles and San Francisco
Cities throughout California previously had their own laws in place governing duplexes, ADUs, and other forms of higher density housing. Some of the more progressive cities in terms of politics and urban planning intended to already allow these kinds of units to some degree, but others as we’ve already discussed imposed sharp restrictions on them.
Those differences can create especially severe problems with housing across the metropolitan area, because the main cities actually govern only a minority portion of the urban areas surrounding them. Los Angeles for example is surrounded by more than 90 different municipalities, each with their own zoning codes. And the San Francisco Bay area isn’t much better, though it does tend to have larger suburbs.
The new laws passed and implemented at the beginning of 2020 changed all of that, though. As such, it’s currently difficult to find clear information about the specific laws that are now in place in each of the cities, because the new laws did make provisions for local areas to make small adjustments to the standards as long as they comply with the big requirements. Most of the cities are currently in the process of rewriting their zoning codes.
What that means is that anybody who wants to build an accessory dwelling unit on their property needs to make sure that they work very closely with their local planning office. There are greater possibilities for construction of additional units now, but the laws are not completely set in stone yet.
Seattle
Seattle is kind of an odd case. It’s far more geographically constrained in the city center than most other metropolitan areas, and their zoning laws are particularly complex.
As of February 2020, though, there are bills working their way through the Washington state legislature that would follow a similar path to California. These would open up single-family neighborhoods to more dense development possibilities similar to accessory dwelling units and also more easily-built duplexes.
As it is now, though, it is possible to find some slightly denser housing in the Seattle area, including town houses, which function like duplexes as long as they conform to specific restrictions. They are single structures with two units that share a wall, side-by-side, rather than being stacked.
Portland
Oregon is another interesting case. The state legislature there passed laws back in the middle of 2019 that had a very similar effect to the new California laws.
California has been somewhat more aggressive about implementation than Oregon has, but the Oregon law might actually be even more radical.
According to OPB,
“Under the new bill, cities of more than 1,000 in the Portland metropolitan area and those of more than 25,000 in the rest of the state will have to allow up to fourplexes in single-family neighborhoods. Cities between 10,000 and 25,000 would have to at least allow duplexes.”
This law has the potential to fundamentally reshape the way cities are built in Oregon. The California law, by contrast, would not change the way most neighborhoods look from the street.
The fact that this is such a radical change, though, means that it is going to be more difficult for the market for housing to fully respond to the shift. There’s a lot of factors that will need to be worked out by developers and local municipalities.
Conclusion
From all of this, it’s clear that governments and builders are working together to alleviate the problems of the housing shortage and high costs that have been plaguing these major American cities for so many years.
It’s also a really important opportunity for homeowners. In these areas, you have the potential to use your property for a great social benefit, and in many cases you can even dramatically increase your income by renting out an accessory dwelling unit on your property.
If you choose to go that route, make sure that you always carefully research your local laws and ordinances before beginning any construction project, and make sure that you get all the necessary permits to ensure that your project is legal and will benefit you for years to come.