Washington’s Density Mandates: Opportunities, Conflicts, and Legal Pitfalls

Overview

Washington State’s slew of recent housing legislation has fundamentally changed how residential property may be developed in cities like Bellingham. In an effort to address housing shortages and affordability, the Legislature has required cities to allow increased residential density in areas that were historically limited to single‑family homes.

While these changes create real opportunities for homeowners and small developers, they also introduce new legal risks, particularly where new public mandates collide with old private covenants, condominium law, short‑term rental regulation, and infrastructure constraints.

This article summarizes two of the most significant statutes now being implemented locally and highlights some resultant legal issues that may emerge in Bellingham and throughout Whatcom County.

The Legislation

HB 1110 – “Middle Housing”

House Bill 1110, codified primarily in RCW 36.70A.635 and RCW 36.70A.636, is commonly referred to as the “middle housing” bill. It requires cities subject to the Growth Management Act to allow multiple residential units on lots that were historically zoned for a single‑family detached home.

Depending on a city’s population and proximity to major transit, lots that once permitted one house must now allow duplexes, fourplexes, or even more units. In practical terms, cities are now largely prohibited from maintaining zoning districts that allow only one detached dwelling.

The stated goal of HB 1110 is to increase housing supply by allowing more diverse and moderately priced housing types such as townhomes, cottage clusters, and multiplexes within existing neighborhoods.

HB 1337 – Accessory Dwelling Units (ADUs)

Similarly, House Bill 1337, primarily codified at RCW 36.70A.680 and RCW 36.70A.681, dramatically expands the ability to construct Accessory Dwelling Units (ADUs), including backyard cottages and basement apartments.

Among other things, HB 1337 requires cities within urban growth areas to allow up to two ADUs per lot, eliminate owner‑occupancy requirements, reduce permitting barriers and caps impact fees and prohibit zoning rules that effectively bar ADUs.

These changes apply broadly across Washington and significantly limit local discretion to restrict ADU development.

Local Implementation in Bellingham

In December 2025, the Bellingham City Council adopted Ordinance 2025‑12‑035 and Ordinance 2025‑12‑032, implementing the mandates of HB 1110 and HB 1337. Together, these ordinances allow infill housing in nearly all residential zones and substantially ease restrictions on the construction and use of ADUs.

As a result, property owners throughout Bellingham can now explore development options that were unavailable just a few years ago.

Conflicts and Legal Pitfalls

Public Mandates vs. Private Covenants

One source of potential confusion that may arise because of all the new legislation involves the interaction between new zoning rules and old private covenants.

In Washington, obtaining a building permit does not override private CC&Rs or HOA restrictions. Many Bellingham neighborhoods remain subject to recorded covenants drafted decades ago, often limiting lots to “one single‑family detached dwelling.”

The law draws an important distinction: Covenants created after July 2023 may not prohibit ADUs or middle housing. Older covenants, however, are generally treated as private contracts and may still be enforceable.

For example, we can easily foresee situations where a property owner has city approval to build a duplex or backyard cottage, but faces resistance or litigation from neighbors or an HOA relying on pre‑existing CC&Rs. Whether those covenants are enforceable can be a trickly legal question.

The “Condo‑ization” of Backyards

HB 1337 also permits ADUs to be separately sold as condominium units, a change that may have significant consequences down the road.

When a homeowner builds a detached ADU and sells it independently, the result is often a two‑unit condominium on a single lot. This “micro‑HOA” would have shared utilities, common areas, and long‑term legal obligations.

In practice, this can create disputes over a number of things. For example, responsibility for maintenance of shared yards or driveways, allocation of insurance coverage and liability, or noise, nuisance, and use of common areas

Transactions where owners do not fully appreciate that selling an ADU converts a private backyard into a shared legal relationship can lead to potential issues and disputes down the road. It will become increasingly important to properly draft governing documents and plan for long-term coownership.

Short‑Term Rental (STR) Conflicts

The intent of these bills was to create long-term housing, but it should come as no surprise that many property owners are using the “no owner-occupancy” rule of HB 1337 to build  ADUs to use as hotel style short-term rentals, such as Airbnbs.

In response, cities like Bellingham have adopted separate laws restricting ADU use as Airbnbs. For example, under Ordinance 2025‑12‑035, detached ADUs generally may not be used as short‑term rentals outside certain commercial, multi‑family, or urban village zones.

This tension is already producing legal disputes. Property owners argue that the state’s mandate to “allow ADUs” implies a right to rent them freely, while cities maintain they retain police powers to regulate commercial lodging.

This area of law remains unsettled and is likely to be litigated further.

Sloppy Legal Descriptions and Title Risk

Washington real estate transactions require precise legal descriptions, and the rapid increase in lot splits, short plats, and condominiumized ADUs has created fertile ground for drafting errors.

This could lead to an increase in deeds, seller‑financed contracts, and lease‑to‑own agreements that fail to accurately describe newly created units or boundaries. These defects can cloud title and may require “quiet title actions” to resolve, which can happen years after construction is completed.

Bottom Line

There are a lot of changes happening in Washington related to zoning and residential construction. They are actively reshaping residential development not only Bellingham and Whatcom County, but across the state.

For homeowners and developers, these changes offer meaningful opportunities. But they also carry legal risks where new public mandates intersect with old private covenants, condominium law, and rental regulation.

Early legal planning, careful drafting, and a clear understanding of both state and local law are increasingly essential in this new landscape.