Recently, I came across a New York Times article published a few weeks ago, A House With an 'Umbilical Cord', that introduced me to a new term: 3-Pack Developments.
The 3-pack in the article refers to three dwellings on a single lot, two of which are connected by a skybridge, serving as the metaphorical umbilical cord.
These types of developments have been gaining popularity in cities with high housing demand and limited space for new construction. They offer a unique solution to maximize land use.
It all has to do with ADUs, or accessory dwelling units, and how developers are creatively maximizing the amount of living space on a single lot.
Their manipulation of the local zoning code is brilliant and could mean opportunity for all of us around the country. However, I'm concerned about the potential impact on the architectural exemptions.
Here's how it all started.
A growing number of US states have enacted laws requiring local governments to allow more than one housing unit on lots previously reserved for single-family homes.
These measures are part of efforts to increase housing supply and address affordability concerns.
The states listed below have passed such statewide reforms since the late 2010s, with an acceleration in activity in the past four years.
- California (2021) - SB 9 allows up to 2–4 units per lot statewide in single-family zones.
- Oregon (2019) - HB 2001 requires mid- and large-size cities to permit duplexes, triplexes, fourplexes, cottage courts, and townhouses on single-family lots.
- Washington (2023) - HB 1110 bans single-family zoning in medium and large cities, requiring cities to allow up to 4 units per lot (with a higher limit near transit/affordable areas).
- Maine (2022) - L.D. 2003 mandates additional housing units and accessory dwelling units (ADUs) on single-family lots statewide.
- Vermont (2023) - The HOME Act (S.100) allows more than one unit on most lots where single-family homes were previously permitted.
- Montana (2023–2024) - Laws adopted after task force recommendations require cities to allow duplexes, triplexes, quadplexes on single-family parcels in many cases.
- Colorado (2024) - Requires cities to allow more multifamily units near transit and bus corridors, reducing bans on multi-unit lots in single-family zones.
- Connecticut (2021) - Broadened ADU legality and limited local exclusionary controls, effectively enabling small-scale multifamily housing across most communities.
- Maryland (Anne Arundel Co.) (2024) - Bill legalizes triplexes, fourplexes, multiplexes, stacked townhouses in certain previously single-family areas, effective July 2025.
- Massachusetts (Cambridge) (2025) - Cambridge ended single-family zoning, permitting multi-unit buildings (up to four stories) in all neighborhoods.
- Texas (Austin) (2023) - Austin now allows up to three homes per residential lot, ending single-family exclusivity citywide.
These reforms are ongoing, and other states are considering expanding or refining their laws.
Local implementation details and additional eligible housing types vary significantly, but the trend toward eliminating exclusive single-family zoning is accelerating nationwide.
In Seattle, WA, before 2019, what used to be a single-family lot allowed one ADU, and the owner of the main house had to live on-site.
Under those restrictions, most ADUs were built by homeowners on their existing lots, for use as guesthouses, studios, or offices.
Sounds like most of America. Right?
Now, Seattle allows a second ADU, but one has to be attached to the primary house.
Since then, developers, rather than individual homeowners, have driven the next phase of ADU construction, creating three-unit compounds to maximize the benefits of these unusual rules.
In creating a 3-pack plan, the 'umbilical cord' refers to a shared laundry space, whereas in others, it may be a library or some other communal use.
Will this become mainstream?
In the jurisdictions where I've worked over the years, I, too, have had to create attached ADU plans to circumvent zoning requirements that limit ADU size and the number of kitchens.
Mostly, they've been on larger lots, where both buildings, particularly the ADU, have been one-story structures.
The new trend in zoning deregulation has infiltrated the urban landscape, and the skybridge is an excellent solution for meeting the intent of the code while maintaining exclusivity and open space on a relatively small and restrictive site.
Given the opportunity, I might have thought of a skybridge, but here's something I'm sure I wouldn't have.
Another ingenious workaround has been to create a mini-condo association, which allows the developer to sell each of the three units individually.
According to the NYT article, purchasers who initially sought a traditional townhouse or condo found that an ADU in a 3-pack was often a better deal.
KUOW, Seattle's NPR radio station, spoke with freelance journalist Jane C. Hu (00:15:01), the author of the article. In the interview, Ms. Hu reports that the zoning regulations may be changing again, eliminating the need for the skybridge.
I love this concept! In Washington State, architectural regulations exempt single-family homes, duplexes, triplexes, and fourplexes from requiring an architectural license, which is a fortunate advantage.
However, laws vary from state to state. For instance, in the state where I reside, only one- and two-family dwellings, along with townhouses, are exempt.
A 3-Pack Development, by definition, consists of a two-family dwelling paired with a detached single-family residence. In this case, each building typically requires a building permit.
The question arises, though—how will local building officials classify these structures if they fall under a condo association? This could lead to varied interpretations.
Navigating these nuances requires continuously staying up-to-date with state laws, monitoring local ordinances, and occasionally engaging in conversations (and debates) with building officials to ensure clarity.
This is precisely why I joined the AIBD 30 years ago (this month). It’s been an invaluable resource for addressing challenges like these, and I know I’m not alone in appreciating all that it offers.
"I have seen the AIBD come against jurisdictions that have tried to create such a condition, successfully. If you wonder why the AIBD has been so important to me, this is a major reason. They have shown, in the 4 years I have held a professional membership, that they will do what is necessary to protect this provision of the law."
Dennis Fletcher, CPBD, Greensboro, MD
A big shout-out to Dennis and all the incredible members of AIBD.
It truly amazes me that not every professional involved in drafting house plans across America is part of this organization.
That said, AIBD has carried the immense responsibility of representing the entire industry—members and non-members alike—since it became a nonprofit association 75 years ago (this month).
I can only hope enough dedicated individuals step up to ensure its legacy continues for another 75 years.
If you'd like to connect with me or get involved, use any of the links below. Let’s keep this mission alive!
Go forth and design boldly,
Steve Mickley, Executive Director
Email: steve.mickley@AIBD.org
Let's chat: AIBD.org/meetsteve
Join: AIBD.org/join-aibd
Volunteer: AIBD.org/volunteer
P.S. - Speaking of Seattle, the city has created "ADUnivers," a webpage focused on promoting ADUs and providing pre-approved plans.
