How Cities and States Can Remove Barriers to ADU Development

Affordable housing is becoming increasingly scarce in cities across the country, and accessory dwelling units (ADUs) are a scalable yet under-utilized housing solution, as was discussed in a previous post. Yet, for all their value, ADUs can be a stubborn process to get started. The average ADU costs $200,000 – not including permitting and construction fees – and property owners are incredibly restricted in what they are allowed to build.

So what do you do when you have a stubborn ass of a problem? First, you get a stick.

The Stick

In most U.S. cities, ADUs are restricted to the brink of their life, and in a few cities, like Chicago, they’ve even been outright banned since the mid-20th century. People saw overcrowding and other social ills plague cities, and they blamed urban density. Then, a fertile mixture of motor vehicle mania and post WWII housing policy helped urban sprawl grow.

Cities aren’t scary anymore; they’re vibrant (and, quite frankly, ridiculously expensive). Planners have learned that blight was actually the result of absent health and building codes, and that low-density urban sprawl is an incredibly unsustainable system. Denser living means cheaper housing and more efficient use of our land and resources. Suddenly, the extra space behind every property looks like valuable real estate. In order to take advantage of that, planners are seeking ways to modernize the zoning regulations that currently restrict ADU construction.


Cities along the west coast have been proactive when it comes to ADU zoning standards. The first major step that cities like Portland, Santa Cruz, and Seattle took to jump-start ADU construction was to loosen the laws restricting which properties they could be built on. ADU owners in Portland were surveyed about the barriers they experienced when trying to construct their ADU; a top barrier was the bevy of regulations restricting ADU development and having to apply for special conditional use permits. Below are four mind-numbingly obvious ADU allowances a city can make to loosen the process:

  1. Allow ADU construction in more zoning areas: The City of Portland now allows ADUs to be built in commercial and central employment zones. Previously, they had only been allowed on single-family residential properties – as is the case in most cities. We allow residential uses to be built above commercial uses; why not allow them to be built behind them?
  2. Allow two-story ADUs: The City of Los Angeles realized that as long as the main structure is two stories, then any accessory structure should be allowed two stories as well. The height allowance made more properties eligible to build appropriately sized ADUs – properties that previously were unable to build single-story structures because of side and rear setbacks limiting the building footprint.
  3. Allow ADU construction on non-owner occupied properties: The City of Santa Cruz removed owner occupancy requirements in exchange for affordability restrictions. Requiring owner occupancy affects the property’s resale value, a major deterrent to development. This also allows rental properties accommodate an ADU.
  4. Allow ADU construction on properties of all sizes: Many U.S. cities restrict ADUs from being built on smaller lots. This means that lots in dense urban neighborhoods are ineligible for ADU construction. This is the easiest zoning standard to change.

Lot Coverage

If an ADU is allowed to be built on a property, its size is usually restricted to a certain percentage of the lot and main structure. The District of Columbia requires that an ADU cover no more than 30% of the remaining buildable lot and be no larger than 35% of the main structure or 450 sq ft – whichever is less. This is considered a typical ADU regulatory standard. The City of Portland is more progressive, however. Portland requires that an ADU cover no more than 15% of the total lot and be no larger than 75% of the main structure or 800 sq ft – again, whichever is less. The easiest way to compare the type of ADU development allowed under each regulation is by visualizing it.

The development comparisons below assume an average city lot of 4,200 sq ft and a 2,500 sq ft two-story main structure.

ADU FootprintADU SizeADU Max Size
WASH525.00                    875.00                    450.00
PORT630.00                 1,875.00                    800.00
TEST630.00                 1,000.00                 1,000.00

In both Washington, D.C. and Portland, lot coverage regulations would allow for a larger ADU than is allowed. Lot coverage regulations ensure ADUs are be built at a scale comparable to the lot and structure. This allows more efficient use of urban land. Why build a 450 sq ft structure on a property that can safely support a structure double that size? The purpose of the static maximum allowable ADU size is to prevent monstrosities, not undermine the scalable regulation.

 FootprintSize (% structure)Max Size (sq ft)
Washington, D.C.30% (Remaining Lot Coverage)35%450
Portland15% (Total Lot Coverage)75%800
Test15% (Total Lot Coverage)40%1000

Both cities are compared to a sample zoning regulation that increases site density (smaller footprint and larger structure size), with a maximum size of 1000 sq ft. These requirements allow for a more efficient use of space.

Zoning requirements regulating ADUs should be loosened to allow structures to be built on more eligible properties and at a slightly larger size. Like any good policy strategy, however, financial assistance should be offered to property owners. Part 2 will investigate the various public and private incentives that could strengthen the ADU market, so stay tuned.

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