Housing & Zoning Terms#

ADU (Accessory Dwelling Unit)#

A smaller, secondary home on the same lot as a main house. Think backyard cottages or basement apartments. Seattle has been loosening ADU rules since 2019 to allow more housing in single-family zones.

Why it matters: ADUs are one of the gentlest ways to add housing in existing neighborhoods without changing their character dramatically. They’re a key part of Seattle’s strategy to add “missing middle” housing.

See also: DADU, Missing Middle Housing

Learn more: Seattle SDCI: Accessory Dwelling Units | Seattle OPCD: Encouraging Backyard Cottages

Design Review#

A public process where volunteer boards and city staff evaluate the design of proposed multifamily and commercial buildings. Seattle has eight geographic design review boards that hold public meetings to review projects exceeding certain size thresholds.

Why it matters: Design review can add 1-2 years to project timelines and significantly affect housing costs. The process gives neighbors a voice in shaping development but has been criticized for delays and subjectivity. Washington’s HB 1293 (2025) requires “clear and objective” design standards, forcing Seattle to overhaul its program.

Displacement#

The involuntary movement of residents from their homes or neighborhoods due to rising costs, redevelopment, or other economic pressures. Often disproportionately affects low-income communities and communities of color.

Why it matters: Displacement is the central equity concern in Seattle planning debates. The city’s rapid growth has pushed many long-time residents out of neighborhoods like the Central District and Chinatown-International District. Anti-displacement strategies – from community land trusts to tenant protections – are now required elements of comprehensive planning.

Learn more: Seattle OPCD: Anti-Displacement Framework (PDF) | The Urbanist: Community Resident Preference Policy

DADU (Detached Accessory Dwelling Unit)#

An ADU that’s a separate structure from the main house – typically a backyard cottage. As of 2019, Seattle allows both an attached ADU and a DADU on most single-family lots.

FAR (Floor Area Ratio)#

The ratio of a building’s total floor area to the size of its lot. A FAR of 1.0 means you can build total floor space equal to the lot size (e.g., a one-story building covering the whole lot, or a two-story building covering half). Higher FAR = denser development allowed.

Why it matters: FAR limits are one of the main tools the city uses to control how big buildings can be in different zones. When people talk about “upzoning,” they often mean increasing the allowed FAR.

See also: Upzoning, Zoning

Learn more: Planetizen: What Is Floor Area Ratio?

Grand Bargain#

The 2015 agreement between Seattle developers, affordable housing advocates, and labor groups that created the framework for Mandatory Housing Affordability (MHA). Developers agreed not to oppose inclusionary zoning requirements in exchange for upzones allowing additional building height and density.

Why it matters: The Grand Bargain was a pivotal political compromise that shaped Seattle’s housing policy for a decade. Understanding it explains why MHA exists, why certain neighborhoods got upzoned, and why affordable housing requirements were set at specific levels (5-7% of units or in-lieu fees).

See also: HALA, MHA

Learn more: Seattle HALA: Director’s Report (PDF) | The Urbanist: A Brief History of MHA

HALA (Housing Affordability and Livability Agenda)#

Seattle’s 2015 comprehensive housing policy framework, developed by a 28-member advisory committee convened by Mayor Ed Murray. HALA produced 65 recommendations aimed at creating 50,000 new housing units over 10 years, including 20,000 affordable units.

Why it matters: HALA was the most significant housing policy initiative in Seattle’s recent history. It created MHA, reformed ADU rules, expanded urban villages, and set the stage for current zoning debates. Many of today’s housing policies trace directly back to HALA recommendations.

See also: Grand Bargain, MHA, ADU

Learn more: Seattle HALA Report 2015 (PDF) | The Urbanist: Top HALA Recommendations

HB 1110#

Washington’s 2023 statewide middle housing law requiring cities to allow duplexes, triplexes, fourplexes, and other “missing middle” housing types in residential neighborhoods. Larger cities near transit must allow up to six units per lot.

Why it matters: HB 1110 is a watershed moment for Washington zoning. It overrides local single-family zoning restrictions that have limited housing supply for decades. Seattle had to comply by June 2025, leading to the city’s biggest zoning overhaul since the 1990s. The law fundamentally changes what can be built in formerly single-family neighborhoods.

Learn more: WA Commerce: Planning for Middle Housing | The Urbanist: Seattle Just Rezoned Entire City

HB 1337 (ADU Reform)#

Washington’s 2023 statewide ADU law requiring cities to allow at least two accessory dwelling units (one attached, one detached) on any residential lot within urban growth areas. The law eliminates many local barriers including owner-occupancy requirements, excessive setbacks, and parking mandates for ADUs near transit.

Why it matters: HB 1337 is one of the most sweeping ADU reforms in any state, making it significantly easier to add “gentle density” across Washington. The law enables homeowners to build backyard cottages or convert garages into housing without many of the restrictions that previously made ADUs expensive or infeasible. Combined with HB 1110’s middle housing requirements, this fundamentally expands what can be built on residential lots.

See also: ADU, DADU, HB 1110

Learn more: WA Legislature: HB 1337 | The Urbanist: ADU and Design Review Reforms Pass

HB 1491 (Transit-Oriented Development)#

Washington’s 2025 transit-oriented development law requiring cities to allow midrise housing near major transit stations. Within a half-mile of light rail, commuter rail, and streetcar stations, cities must permit at least 3.5 Floor Area Ratio (FAR). Within a quarter-mile of bus rapid transit stops, cities must allow at least 2.5 FAR. The law includes affordability requirements (10% of units at 60% AMI or 20% at 80% AMI) paired with a 20-year property tax exemption.

Why it matters: HB 1491 is one of the largest state-mandated upzones in U.S. history, requiring significant density increases around every major transit station in the Puget Sound region. The law ensures that billions invested in Link light rail expansion will be paired with housing capacity around stations – addressing past failures where transit investments didn’t produce expected ridership or housing growth. Compliant buildings must maintain affordability for 50 years.

See also: TOD, FAR, ST3

Learn more: WA Legislature: HB 1491 | The Urbanist: State Senate Greenlights TOD Housing Bill

Lot Coverage#

The percentage of a lot that can be covered by buildings and structures. Seattle’s neighborhood residential zones typically have lot coverage limits of 35-50% depending on the zone and housing type.

Why it matters: Lot coverage limits, along with setbacks and height limits, determine how much building can fit on a site. When you hear debates about building “too big” for a lot, lot coverage is often part of the equation. Recent zoning changes have adjusted lot coverage rules to accommodate middle housing.

See also: FAR, Setbacks, Zoning

Learn more: Seattle SDCI: Lot Coverage Standards (Tip 220) | The Urbanist: Seattle Just Rezoned Entire City

MHA (Mandatory Housing Affordability)#

Seattle’s program requiring developers of commercial and multifamily buildings to include affordable units or pay into a fund for affordable housing. Passed in 2019 alongside upzones in many neighborhoods.

Why it matters: MHA is Seattle’s main mechanism for connecting new development to affordable housing production. It’s controversial – developers say it raises costs; advocates say it doesn’t go far enough.

See also: Upzoning, Affordable Housing

Learn more: Seattle Office of Housing: MHA Program | Seattle SDCI: MHA Code

Missing Middle Housing#

Housing types between single-family homes and large apartment buildings: duplexes, triplexes, fourplexes, townhouses, courtyard apartments. Historically banned in most Seattle residential zones.

Why it matters: Seattle’s 2024 comprehensive plan update and related zoning changes are opening up much of the city to missing middle housing for the first time in decades. This is one of the biggest zoning shifts in the city’s history.

See also: ADU, Upzoning

Learn more: The Urbanist: Seattle Just Rezoned Entire City | Sightline: Seattle Deserves a Better Comp Plan

Neighborhood Residential (NR)#

Seattle’s new zoning designation that replaced “single-family” zones in 2021. NR zones (NR1, NR2, NR3) now allow various housing types including duplexes, triplexes, and other middle housing following HB 1110 implementation.

Why it matters: The name change was initially symbolic, but NR zones now permit significantly more housing than old single-family zones. NR1 allows the most density (smallest minimum lot sizes), while NR3 is the least dense. Understanding NR zoning is essential for following current development patterns and zoning debates.

Learn more: Seattle OPCD: Neighborhood Residential Zoning | The Urbanist: Seattle Just Rezoned Entire City

Parking Minimums#

Zoning rules that require developers to build a minimum number of off-street parking spaces. Seattle eliminated parking minimums near frequent transit in 2012 and expanded reforms in 2018, but requirements remain in lower-density neighborhoods.

Why it matters: Parking minimums add significant costs to housing construction (roughly $30,000-$50,000 per space for structured parking). Studies found Seattle’s 2012 reforms resulted in 18,000 fewer spaces built, saving over $500 million. Washington’s 2025 parking reform law (SB 5184) now caps how much parking cities can mandate statewide.

SB 5184 (Parking Reform)#

Washington’s 2025 statewide parking reform law that caps the minimum parking requirements cities can impose on new development. Cities cannot require more than 0.5 parking spaces per residential unit. The law also prohibits any parking minimums near frequent transit, for senior housing, childcare facilities, small units, and existing buildings undergoing a change of use.

Why it matters: Parking mandates have been one of the biggest hidden costs in housing development, adding $30,000-$50,000 per stall for structured parking. By capping how much parking cities can require, SB 5184 allows developers to build more housing on the same land at lower cost. This is one of the most comprehensive statewide parking reforms in the country and directly addresses a key barrier that has inflated Seattle-area housing costs for decades.

See also: Parking Minimums, TOD

Learn more: WA Legislature: SB 5184 | The Urbanist: Parking Reforms Become Law

Setbacks#

Required distances between buildings and lot lines (front, side, and rear). Seattle’s neighborhood residential zones typically require 20-foot front setbacks, 5-foot side setbacks, and rear setbacks of 20% of lot depth or 25 feet (whichever is less).

Why it matters: Setbacks, along with lot coverage and height limits, determine where and how big you can build on a lot. Recent zoning reforms have adjusted setbacks to make middle housing more feasible – for example, reducing front setbacks to allow “Seattle Six” configurations with stoops and covered porches.

See also: Lot Coverage, Zoning

Learn more: Seattle SDCI: Lot Coverage, Height and Yard Standards (Tip 220) | The Urbanist: Seattle Just Rezoned Entire City

Single-Family Zoning#

Zoning that only allows one house per lot. Seattle had roughly 75% of its residential land zoned single-family until recent reforms. The city has been gradually renaming and reforming these zones (now called “Neighborhood Residential”) to allow more housing types.

Why it matters: Single-family zoning is the legacy framework that restricted density across most of Seattle. Understanding its history and recent reforms is essential for following current zoning debates.

See also: Missing Middle Housing, Upzoning

Learn more: Seattle OPCD: Neighborhood Residential Zoning | Cascade PBS: Seattle shares plan for more housing density

Stacked Flats#

A middle housing type consisting of dwelling units stacked vertically in a building of no more than three stories, where each unit occupies a single floor. Unlike townhouses where units are side-by-side, stacked flats place units on top of each other, often sharing a central entrance or having individual street-level entries.

Why it matters: Stacked flats were historically common in Seattle but became difficult to build due to zoning restrictions and financing challenges. Washington’s HB 1110 explicitly includes stacked flats in the definition of middle housing that cities must allow. Seattle’s 2025 zoning reforms created incentives for stacked flats, including FAR bonuses (up to 1.4 on larger lots near transit). Because stacked flats can provide more units than townhouses on the same lot size, they’re a key housing type for meeting density goals while maintaining neighborhood scale.

See also: Missing Middle Housing, HB 1110, FAR

Learn more: WA Commerce: Middle Housing Model Ordinances | The Urbanist: The Deck is Stacked Against Stacked Flats

Upzoning#

Changing zoning rules to allow larger or denser buildings than previously permitted. Can mean increasing building height limits, FAR, or allowed unit counts.

Why it matters: Upzoning is one of the most debated topics in Seattle land use. Proponents say it’s essential for housing affordability and climate goals. Opponents worry about neighborhood character, displacement, and infrastructure strain.

See also: FAR, MHA, Single-Family Zoning

Learn more: The Urbanist: Will Seattle Embrace Multiplexes? | Planetizen: Seattle Changes Single-Family Zoning Name

Affordable Housing#

Housing where residents pay no more than 30% of their income on rent/mortgage. In Seattle policy, “affordable” usually means affordable to households earning a percentage of Area Median Income (AMI) – often 60% or 80% AMI.

Why it matters: Seattle’s AMI is high (~$120k for a family of four), so “affordable at 80% AMI” still means rents that many people can’t afford. Understanding the AMI benchmarks helps you evaluate whether affordable housing proposals will actually serve the people who need them.

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