This article will explain how side and rear yard setbacks are preventing the development of ADUs, and provide a prescription for how to fix these common standards. In part I, I’ll review the appropriate intervention as a legislative matter in the broader context of the ADU movement, and give guidance that will hopefully trigger appropriate changes to municipal codes. Then, in part II, I’ll explore other facets of ADU setbacks.

Part I ) Strong ADU Legislation Reforms Sometimes Overlook the Details

The State of Statewide ADU Setback Regulations

As of October 2024, there are seven states that have adopted strong ADU reforms:

1. California

2. Oregon

3. Washington

4. Colorado

5. Montana

6. Arizona

7. Massachusetts

By “strong” ADU legislation, I mean laws that align with AARP’s ADU model ordinances. Of those seven states, only California and Oregon's laws have gone into effect already, but the other states will be enacting their ADU laws in the coming months and years.

However, as useful as the AARP's state model ordinance is as a metric for enacting strong ADU laws, their guidance does not detail the finer technocratic development regulations that are critical to actually initiating a bonafide ADU movement. 

This post delves into one of those remaining regulations not addressed by the model ordinance in finer detail - rear yard and side yard setbacks. It will give context for how rear and side setback standards tend to kill ADUs, even in jurisdictions with otherwise strong ADU reforms. 

Of the states with strong ADU reforms, here are the legislative standards in place for side and rear yard setbacks:

1. California: 4 ft setbacks

2. Oregon: Silent about setbacks

3. Washington: Defaults to “underlying zoning” for setbacks

4. Colorado: Defaults to “underlying zoning” for setbacks

5. Montana:  Defaults to “underlying zoning” for setbacks

6. Arizona: 5 ft setbacks

7. Massachusetts: Silent about setbacks

While California and Arizona’s ADU reforms compel 4 ft and 5 ft setback maximum for rear yards and side yards respectively in subject jurisdictions, the other five states that have passed strong ADU reforms DO NOT effectively address side and rear setbacks.

Therefore, despite the fact that the ADU laws in the other four states are fairly strong, it may nonetheless prove impossible for homeowners to build detached ADUs within many cities in those states, due to onerous default setback requirements. 

In a truly liberalized ADU market, 2/3rds to 3/4ths of new ADUs are built as detached ADUs; this is what the market tends to create for a variety of reasons. 

Given that most ADUs are developed in higher-cost urban areas–central city neighborhoods- the lots that host ADUs tend to be on the smaller side. 

Lots in urban areas tend to be older and smaller, largely established prior to automobile domination. Conversely, residential properties established in the mid and latter part of the 20th century tend to be larger lots on the suburban and exurban fringe of central cities. 

Based on the cities where ADUs are actually being developed in greater quantities, it is fair to assess that more ADUs are built on smaller urban lots (5,000 - 7000 sq ft) than on larger suburban lots (7,000 - 20,000 sq ft).

Therefore, when establishing statewide laws that will impact the cities where ADUs will actually be built, it is important that the zoning laws are written such that it will actually allow detached ADUs to be developed in these places. If the laws do not help achieve that outcome, then they have somehow missed the mark.  

This post reviews how jurisdictional codes are missing a critical mark in many cases - and accidentally preventing any substantial ADU uptick. 

How Rear and Side Setbacks Hinder ADUs 

To illustrate how rear and side yard setbacks work, we’ll switch gears from text and use diagrams instead. 

For the reasons described above, we will use a standard urban lot. This lot is 5,000 sq ft. It is 50 ft wide and 100 ft deep. 

We’ll position the typical primary house built on that typical property 20ft from the front street lot line, to comply with typical front yard setback requirements. 

Here's an aerial image of six standard 50 ft by 100 ft lots with typical homes in Portland, Oregon.

This first diagram below shows a typical lot, mirroring the massing of the six lots above. It shows front yard setbacks, the primary dwelling, and a 6 ft fire separation between the primary dwelling and the potential buildable area in the back yard. Notwithstanding setbacks, about 35% of the property is left in which to place a detached ADU in the backyard… somewhere.  

In these diagrams, each background grid box equals 10 ft. The diagrammatic property is shown as being 10 boxes long, and 5 boxes wide. 

Now, let’s show the potential buildable area once 5ft setbacks have been applied. This buildable area is fairly constrained, but there’s sufficient room to fit a good-sized one-story ADU in any number of possible configurations within the buildable area confines. 

Next, let’s show the potential buildable area of that same property if 10 ft setbacks were applied. We’ve suddenly shrunk the potential size of the ADU from over 1,000 sq ft, to only 570 sq ft (19 ft x 30 ft). 

As a point of reference, 570 sq ft is just barely large enough to accommodate a reasonable 1BR ADU. 

Next, let’s show the potential buildable area if 15 ft setbacks were applied. We’ve now shrunk the potential size of the ADU from 570 sq ft to 280 sq ft. 

ADUs this size make very little economic or practical sense to develop. I’ve visited over one thousand ADUs in the last 15 years, and only seen three ADUs this size. 

Next, let’s show the potential buildable area if 20 ft setbacks were applied. We’ve now shrunk the potential size of the ADU to a maximum buildable area of 90 sq ft. Given that that minimum size bathroom is ~35 sq ft, this would leave approximately 55 sq ft for the living space. A queen bed would take up 33 sq ft. The kitchen counter with a fridge, stove, and sink would take up more than 20 sq ft. 

There wouldn’t be any space left to actually stand in an ADU of this size, let alone a place to practice yoga or do the hokey pokey. 

Legislation that Defaults to Underlying Setbacks

In the absence of designating prescriptive setbacks that preempt local zoning, legislation allows jurisdictions to use their existing setback requirements for ADUs. As described previously, four of the seven states that have “strong ADU reforms” nonetheless allow jurisdictions to default to their own jurisdictional setback standards for ADUs, whatever they are. 

Cities that have given thought to ADUs may have crafted conducive setback requirements for them. However, municipalities usually default to the “underlying zoning” standard when regulating side and rear setbacks on a property. The underlying zoning standards were originally intended to define the buildable area for the primary dwelling. 

Since there is ample space even on small lots for developing primary dwellings, it was likely never a challenge to “respect the setbacks”. 

Here are examples of cities in states with “strong ADU reforms” and their “underlying zoning” requirements for rear and side yard setbacks, which carry over to detached ADUs. 

• McMinville, Oregon -            20 ft rear yard setbacks

• Wheat Ridge, Colorado -     15 ft rear and side yard setbacks

• Westford, Massachusetts -   30 ft rear yard setbacks, and 15 feet side yard setbacks 

• Weston, Massachusetts -     20-45 ft rear yard setbacks

• Stow, Massachusetts  -        40 ft rear and side yard setbacks

These types of setback requirements are unfortunately not the exception. They are the rule. It is likely that your city also has similarly onerous setbacks unless it has specifically viewed this matter through the ADU developer prism of “reasonable” regulations. 

Here’s the setback table for McMinnville, Oregon, for example.

Take an average lot in McMinnville, and try to figure out how to place a detached ADU while respecting the 20 ft rear yard setback required. In the typical property shown below, the backyard is only 28 ft deep. 

Given the 6 ft fire separation requirement from the primary structure described above, that leaves 2 ft of yard depth to work with. I’m not an architect, but 2 ft of total building length seems not only limiting; it makes detached ADU development impossible. 

This setback’s impact on buildable area in a backyard for a detached ADU is really a simple matter of geometry, with a dash of development economics and a tablespoon of common sense. 

Setback standards are likely just a legacy standard for primary homes established by planning staff, long before they ever considered the requisite geometry of building a detached ADU.

Current planners may have never given thought to how their zoning code is institutionally hindering the development of ADUs because they “rarely see ADU applications”. 

Gee, I wonder why McMinnville, Oregon has issued fewer than 20 ADU permits issued in the last 5 years despite the strong statewide ADU reforms that came into effect.

Connecting the dots? 

Once the underlying logic here is communicated to planning staff, it should be extremely clear to them. 

The good news for ADU advocates (e.g., a homeowner who wants to build an ADU) is that correcting onerous setback standards for ADUs will not be nearly as politically controversial as eliminating the three top poison pills for ADUs - the ADU regulations that are directly addressed in AARPs’ model ordinance.  

If a city or a state claims to want ADUs as a policy matter for more housing choice, it must institute “reasonable” setback standards for detached ADUs. 

A “Reasonable” Setback Requirement for Detached ADUs is 5 ft

I’m not dogmatically fixated on 5 ft; 4 ft is fine, and some jurisdictions go down to 3 ft. Heck, Portland goes to 0 ft setbacks for small ADUs. And 6 ft is not the end of the world. 

But, in practice, most rear yard and side yard setback standards tend to be set in 5 ft increments: 5 ft, 10 ft, 15 ft, 20 ft, etc. And, of those, 5 ft setbacks is the only increment that actually allows for the development of detached ADUs. So, I’m throwing out there that moving forward for the ADU movement, the best practice for ADUs are 5 ft rear and side yard setbacks. 

Here's the setback table from the zoning code for Portland, Oregon as a point of reference.

If your city defaults to the underlying zoning, it’s very likely that you are being stymied from building an ADU on your property. So, speak up. Advocate to put those side and rear yard setbacks where they belong. 

That concludes my policy intervention on side and rear yards setbacks. This explanation should hopefully tactfully and delicately inform policymakers across the country about how to get setbacks corrected to actually allow for ADUs in real life. 

But, for those of you who know me and my obsession with ADU matters, you will not be surprised to learn that this is actually just the beginning of a deep dive into a number of other setback considerations ADUs. 

Don’t bother reading further unless you want to get into more specifics about ADU setbacks. 

Part II) Getting Deep in ADU Side and Rear Yard Setbacks

The Evolution of Setback Regulations in America

Early setback regulations emerged in 1799 Hartford, Connecticut, not for the reasons we commonly think of today, but primarily to allow for future street expansion. These initial regulations were treated as eminent domain actions, requiring the government to compensate property owners for the restricted land use. 

Here’s a thought experiment: Imagine if local governments had to compensate homeowners for the land takings represented by setbacks. Setbacks would shrink to 0 ft overnight.

A significant shift occurred in the 20th century with the advent of comprehensive zoning. Setback requirements evolved from an eminent domain tool into a police power mechanism, allowing local governments to regulate property use for public benefit without compensation. 

This change established setbacks as a standard feature in modern zoning codes. Today's setback regulations serve multiple purposes:

Health and Safety

• Enable proper light and air flow between buildings

• Create fire breaks between structures

Quality of Life

• Provide privacy between neighboring properties

• Create space for outdoor activities and gardens

Community Character

• Maintain neighborhood aesthetics

• Prevent overcrowding

• Preserve open space

While these regulations continue to shape our cities, there is debate about their effectiveness and appropriateness in modern urban development, particularly as cities face increasing density and housing affordability challenges. 

For a richer discourse on the context and purpose of setbacks in the US, I had an AI put together a pretty darn entertaining 23 minute podcast on the topic. Auditory learners will really enjoy this. 

Legislative Nuance about Side and Rear Yard Setbacks

Two story ADUs:

One way to deal with onerous setbacks is to build more square footage on the 2nd story of an ADU, rather than squeezing all of the square footage into the ground level. 

Unfortunately, some places don’t really allow 2 story ADUs. For example, California’s “state-exempt ADU”--that is, ADUs that are exempt from local zoning requirements- limits them to 18 ft in height, which is inadequate to accommodate a 2-story ADU reasonably. Instead, the height standard must be 24 ft to reasonably allow for a two-story ADU measured to the peak of the roof. 

Furthermore, given that cities and states want more accessible housing for aging in place - a primary policy goal for ADUs in general - the zoning code should be written to allow for a reasonably sized detached ADU to be entirely built on ground level. A reasonable one-story ADU would be at least 600 sq ft for a 1-bedroom ADU, or 800 - 1,000 sq ft for a 2BR ADU. Thus, the setbacks should allow for “adequate buildable area” for “typical lots” to comfortably accommodate that structural footprint. 

Tiered setbacks depending on the size of the lot:

The simplest way to write the setback standard is to say, “5 ft”. However, if there’s opposition to this uniformity, it is worth mentioning that as the size of a lot increases, it is easier to respect larger setback standards. 

For a 20,000 sq ft lot or larger, as is found in many suburban areas, a 10 ft setback would not necessarily stop the development of detached ADUs. So, a tiered setback standard based on lot sizes may be a politically expedient workaround to dealing with civic opposition to smaller setbacks. 

I’d personally favor sticking to a 5 ft setback for simplicity, but a tiered standard is a reasonable method to handling this issue politically, if needed. 

Penalizing smaller lots with 10 ft or greater setback standards is regressive.

Similar to stating that the ADU may only be up to 25% or 50% of the size of the primary house (another common and equally problematic ADU standard), it is also regressive to apply a uniform 10 - 20ft setback standard based on larger lot dimensions, to smaller lots. You can read more about this here. 

In summary, for smaller lots, there’s a proportionally less buildable area with uniform setbacks, due to laws of geometry. And my sources tell me that math is stubbornly immutable. 

While a 10 ft setback is acceptable on a 20,000 sq ft lot, it most likely kills the potential for an ADU on a smaller 5,000 sq ft lot. 

Gold-standard ADU codes compel planners, building officials, and high-performance energy standard keepers to put their ADU developer hat on, so that they do not accidentally institute regressive policies that make ADU development impossible. 

Plain language, pretty please???

/open_rant

Don’t get me started, but can City planning departments please make it easier to figure out the setbacks and other basic development standards for detached ADUs on a dedicated web page?

As it stands, for most cities, you have to be a professional architect or developer to determine what zone you’re located in based on a crude PDF zoning map or a buggy ESRI interactive map, then refer to a zoning lookup table in an entirely different part of the zoning code in non-OCR-ed PDF. Or, even worse, you’re stuck in a Municode zoning abyss, unable to navigate to the correct sections through its outdated 1990’s user interface. 

See if you can figure out the rear yard setbacks for an ADU in a neighboring jurisdiction whose website you don’t already know intimately, and you’ll understand what I mean. 

95% of jurisdictions do an awful job of providing basic ADU regulations in plain language in a single place. 

It’s as though municipal planning departments are deliberately trying to make the discovery process difficult for citizens. 

Planning staff should aim to help average homeowners who wish to build ADUs by making basic information easy to access. Provide a simple web page with the ADU standards. 

This project would take roughly 1 hour of staff time. Hire me. I’ll do it. No excuses. Get it together, man. 

/end_rant

Institute form-based setback codes for detached accessory structures. 

In 2016, I sat on a citizen advisory board in Portland to help create better detached ADU development standards. 

At that time, 2-car garages were already allowed to be located within the 5 ft setback. But other detached accessory structures, including detached ADUs, had to respect a 5 ft setback. 

Informed by a form-based code philosophy, Portland adopted a uniform setback standard for all “detached accessory structures”, regardless of their use. 

Henceforth, Portland would treat garages, sheds, and detached ADUs the same, rather than having more permissive setback standards if the structure were a garage or shed than if it was a structure used as living space for a person. 

This was a laudable outcome and helped me to understand how codes are often biased towards housing cars instead of people; which is kind of terrible when you think about it. 

It’s the definition of inhumane. 

Who are we building our cities for, after all?

I’m not blaming anyone here. It’s probably just historical oversights and thoughtlessness that led to zoning biases towards cars. 

The bottom line is that jurisdictions should audit whether they are implicitly biasing their zoning code to induce automobile use, rather than inducing more affordable housing choices.

Within the 5 ft setbacks, allow condenser units for ductless mini-splits.

Setbacks are often wasted space, so we should be able to place the condenser units for ductless minisplit HVAC systems in those setbacks. This is important because condenser units are sizable, and it’s difficult to find a good spot for them that isn’t an eyesore in the remaining small back yard spaces, if they can’t be located in the side and rear setbacks. 

Practical Advice for Homeowners Regarding Side and Rear Setbacks 

It’s expensive to authoritatively determine side and rear property lines.

Property lines are notoriously difficult to nail down precisely. Obvious markers such as sidewalks, walls, and fences do not reliably align with actual real property lines. 

Locating survey monuments or pins to determine where a foundation can be placed often requires a surveyor. This can cost $2K - $5K. 

There are several tricks that homeowners can use to figure out roughly where their official property lines are located. These include measuring the length and width of a property from other notable markers along the front property line, such as other clearly delineated property corners, or using a metal detector to find legacy survey pins in the property corners. 

But, if you plan to build an ADU right up to the setbacks, as most homeowners do, then plan to hire a surveyor to authoritatively denote property lines with survey pins. 

If you wish to avoid paying for a surveyor for this service, you could shift the detached ADU far enough away from the property line that there’s no question from the building inspector about whether the ADU is sufficiently respecting the rear and side yard setbacks. 

However, it’s typically in a homeowner’s best interest to build right up to the required rear and/or side yard setbacks to retain as much of their remaining yard space as possible. And, building up to the setbacks will likely require hiring a surveyor so that the building inspector can sign off on the ADU foundation form placement before the actual concrete pour. 

Determine the method by which your jurisdiction measures the “outside of buildings”.

In an ideal world, cities would measure from the property line to the ADU foundation wall to determine whether an ADU is “respecting the setback”. 

But some jurisdictions may measure instead to the siding, or worse still, to the closest overhanging eave. These distinctive measurement methodologies will affect where you can place a detached ADU, relative to the side and rear setback. 

There is no uniformity of measurement methodologies across jurisdictions here, so you’ll have to inquire about this directly, or rely on an ADU savvy architect who knows this stuff. Similar methodological differences exist for measuring building height, square footage, and lot coverage. 

Enhancing the Utility of Requisite Side and Rear Yard Setbacks

There are ways to effectively utilize a “wasted space” that is created by setbacks. Namely, while you can’t build “structures” within the setback, you can tactfully use pavers, gravel, pavement, or landscaping to enhance the utility of the setback space. 

Unless the zoning code specifically prohibits it, you can even place the entry doorway to the ADU along the building facade that abuts the 5 ft setback, as illustrated wonderfully here. 

This is a dreamy 5 ft side yard setback. Dwell Magazine should do an issue on dreamy side yard setbacks. 

Alternatively, you can simply store ladders or garbage cans in the setback. Just don’t do what I did and let your setbacks go to the weeds!

Next Generation Rear and Side Setback Standards for Alternative Infill Housing

Portland allows for an occupied RV on residential property.

Here are the regulatory standards. Among the many cool things about using tiny homes on wheels as a dwelling, they are not subject to rear and side yard setbacks. 

Just as a vehicle is not subject to rear and side yard setbacks, neither is an inhabited RV. Again here, Portland embraced a form-based code method of zoning, just as they did with the 2016 accessory structure zoning code update described previously. 

The setback for a detached structure should be treated the same whether it houses a car or a person. 

And, an RV should be allowed to park where the zoning code allows it to be parked, whether it’s vacant or occupied. 

Note how this tiny house on wheels (THOW) is tucked just a foot away from the concrete structure on the right. Now that’s urban infill! 

Similarly, because it’s on wheels, a THOW can be parked under a tree canopy. Conversely, an ADU has a foundation, and therefore may have to contend with “tree root protection zone” standards, because building foundations can kill tree roots which can ultimately kill the tree itself. 

These seemingly minor regulatory differences substantially increase the flexibility of mobile dwellings over affixed structures like ADUs. I’ve started a general contracting company that installs RV hookups on residential properties, so I see how this plays out regularly first-hand. Most of the inhabited RVs end up a couple feet from property lines.

THOWs offer unparalleled opportunities to create much-needed affordable housing in cities without any government subsidy. It’s a real shame that they’re not allowed to be inhabited just about anywhere due to zoning ordinances. 

Again, RVs are allowed as inhabited dwellings on residential properties. The innate regulatory advantages/bias of being treated as a vehicle instead of being treated like a house becomes increasingly apparent when considering placement of an RV compared to placement of an ADU. 

Subdividing without Subdividing

The subdivision of multifamily, cottage clusters, multiple ADUs, through condoization and through fee-simple partitioning, can start to disaggregate housing from the bureaucratic norms we’ve established for setbacks. 

As we rethink how a “‘single family property”’ can be developed with multiple dwellings using the creativity and freedom afforded through middle housing zoning laws sweeping the country, the setbacks play a less prominent role in how the buildable area can be used. The buildable area of a middle housing lot has no property lines, and therefore, it has no property line setback requirements. 

Multiple buildings placed within the buildable area can be oriented to create courtyards, and can be placed at angles, and the rectilinear setbacks along the property edges simply serve as the canvas boundary for the development. 

As middle housing legislation enables more cottage cluster housing, pocket neighborhoods, planned unit developments, and micro-HOAs of ADUs, it could eventually foster shared outdoor spaces within a platted residential property. 

These inner block developments are common in old European blocks. Despite not having codified setbacks, inner block spaces are truly designed for neighbors, not for cars. What a concept. Everything old is new again.