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Types of Affordable Housing

This page introduces different housing and building techniques that can be used to construct housing units at lower cost that are affordable to more households.

It is part of MRSC's series on Affordable Housing.

Overview

Two of the major factors affecting the cost of constructing a home are size and cost of materials. Advancements in off-site construction and mass timber can make housing more affordable, as well as leading to innovations in small living. All construction in Washington State must follow building standards developed by the State Building Code Council, including the International Building Code and International Residential Code. All housing types and techniques mentioned below are currently allowed by Washington State building standards.

Affordable Housing Types and Models

This page discusses housing types that are affordable to more people in the local real estate market. Many of these housing types have smaller footprints that lower construction costs. Similarly, advancements in off-site construction and mass timber can make housing more affordable, as well as leading to innovations in small living. 

This page reviews several affordable housing types, including micro units, co-living, single-resident occupancy, manufactured homes, tiny homes, tiny homes on wheels, and accessory dwelling units. Micro units, co-living, or single-resident occupancy (SRO) homes can often be found connected to or within existing multi-person buildings or formal shared living spaces. Accessory dwelling units (ADU) can be either attached or detached to other dwellings, while tiny homes, tiny homes on wheels (THOW), and manufactured homes can be considered as detached, independent types of housing. 

Micro Units, Co-Living, and Single-Resident Occupancy 

Micro units generally measure under 400 square feet and are included as part of a multi-unit building. Some micro units include their own private bathroom and a kitchenette, whereas others only include a private living space with shared kitchen and bathrooms. Other units exist somewhere along this spectrum. No matter the layout, these units are generally created in urban development areas, mixed-use zones, or along transit routes to create density and affordability for single residents or small families. Many cities in Washington have begun to allow this type of development.

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These units can also be referred to as: “micro-apartments,” “small efficiency dwelling units,” “micro housing,” “apodments,” “residential suites,” and “co-living apartments,” all with similar requirements and regulations. There is no agreed-upon definition or regulation standard statewide, and not all municipalities specify size limits, but these apartments tend to be between 120 and 400 square feet.

In 2024, the Washington State Legislature passed SHB 1998, which requires certain cities and counties to permit co-living housing on any lot located within urban growth areas that allows at least six multifamily residential units. It also prohibits a city or county from imposing certain regulations on co-living housing.

In some municipalities, small apartments that do not have private bathrooms and/or kitchenettes are regulated under congregate housing or single-room occupancy (SRO) regulations. This type of housing generally includes private bedrooms attached to a hallway with a shared communal bathroom, kitchen, and living facilities. While the line between micro units and single-resident occupancy is thin, there are some differences:

  • Micro units (sometimes referred to as micro-apartments, co-living, “apodments," etc.) are attached to a multi-unit development and may or may not offer completely private living spaces, including a bathroom and/or kitchenette.
  • Single-resident occupancy (SRO) are units where each person living onsite has private living quarters (usually just one room that serves as a combined bedroom/sitting area) but shares a bathroom, kitchen, and other communal spaces.

Examples of Micro Unit, Congregate, and SRO Regulations

Resources for Micro Units, Co-Living, and SROs

  • Urban Land Institute: The Macro-Views on Micro Units (2015) – Compares different definitions of micro-apartments nationally and worldwide; contains lots of great examples.
  • OneSharedHouse 2030 – This research project shares what people say they wanted (and did not want) in a co-living facility.
  • ShareNYC (2018) – The City of New York launched ShareNYC, an initiative aimed at creating co-living and other shared housing developments to meet housing needs.

Accessory Dwelling Units 

Cities and counties (unincorporated urban growth areas) planning under the Growth Management Act (GMA) must allow the development of Accessory Dwelling Units (ADUs), which are small, self-contained residential units located on the same lot as an existing single-family home. These local governments must revise their ADU regulations, within six months after their periodic update due date, to meet a number of requirements in RCW 36.70A.680-.681.  

While the high cost of constructing detached ADUs may preclude them from being considered “affordable housing,” allowing for separate, attached ADUs within existing residences will likely result in new, more affordable housing units. Washington’s senior citizen and disabled persons property tax exemption and qualification for deferral programs also apply to one detached accessory dwelling unit. For more information, see MRSC’s page on Accessory Dwelling Units (ADUs).

The image below shows the different types of ADUs, from detached to attached to (or located within) the main dwelling. 

Accessory-Dwelling-Units

Manufactured Homes and Manufactured Home Parks

The requirements for a "designated manufactured home" is provided for in RCW 35.63.160, including that a designated manufactured home must include at least two sections. Further, when installed, the state requires that a manufactured home be set on a permanent foundation in the manner specified by the manufacturer. Local governments can regulate the siting of manufactured homes and mobile home parks. Skagit County’s handout on Installing Manufactured Homes offers information on county requirements.

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Manufactured homes and manufactured home park zones can offer affordable housing in any local jurisdiction. However, rising land values and new development can force many of these parks to close. Some municipalities, such as Bothell and Kenmore, have instituted regulations for the purpose of preserving mobile home parks in their jurisdictions. For additional information, see our Manufactured Housing Regulation and Preservation page.

  • Bothell Municipal Code Sec. 12.04.100 – Mobile Home Park Overlay zoning classification is used to retain manufactured home parks as source of affordable single-family and senior housing. Applied to existing parks that contain rental pads. Limits development unless a comprehensive plan amendment is adopted.
  • Kenmore Municipal Code Sec. 18.50.140 – Sets standards for existing manufactured housing communities. Specifically states that they shall continue to operate according to the standards that were in place at the time that the parks were approved. Sec. 18.50.150 outlines development standards for new manufactured home communities. Notably, (B)(3) states that they shall be eligible to achieve maximum density permitted by providing the affordable housing benefit.

Tiny Homes on Wheels and Tiny Houses/Small Houses

 When most people talk about “tiny homes,” they are likely to be thinking of a Tiny Home on Wheels (THOW). THOWs are designed to be easily transported from location to location, are often custom built, and usually make provisions for living, sleeping, cooking, and sanitation. But they are not constructed to be used as a permanent dwelling unit and usually cannot meet Building and Residential code requirements (i.e., IBC/IRC). Instead, they are more like Recreational Vehicles (RVs) and designed to be used as temporary living quarters for recreational camping, travel, or seasonal use. 

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Photo courtesy of Portland Alternative Dwellings.

In comparison, a tiny house/small house will vary in size (typically less than 600 sq. ft.), is built on a foundation, and has been constructed to meet Building and Residential Code (i.e., IBC/IRC) standards for a permanent dwelling unit meant to be lived in year-round. Within urban areas, tiny houses/small houses are sometimes used as Detached Accessory Dwelling Units (DADU) and located in backyards of larger primary residences. However, they have also been developed in clusters (as stand-alone communities) or used as primary residences in rural or rural/suburban settings. 

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In 2019, the Washington State Legislature expanded the potential for construction of tiny homes on foundations to make it more feasible for these structures to be used as primary residences. Tiny homes on foundations are also being used in cities and counties to temporarily house unsheltered individuals. This sometimes leads to the creation of tiny home villages (see the Low Income Housing Institute, which operates several villages) where each resident has an independent tiny home residence, as well as shared cooking and sanitation facilities.

Examples of Tiny House/ Small House Regulations

  • Clark County Code Sec. 40.100.070 – Includes tiny houses in its definition of single-family dwellings as detached dwellings of less than 150 square feet, constructed or mounted on foundations and connected to utilities. Clark County groups tiny houses with other single-family homes, with both housing types being permitted in similar zones.  
  • Langley Municipal Code Sec. 18.22.290 – Allows for the creation of tiny homes clusters surrounding a common open space, subject to specific standards.

Examples of THOW, PMU and RV Regulations

A park model home/unit (PMU) is built according to recreational vehicle (RV) industry code and follows the same rules for quality and design. However, a PMU is mounted on a trailer and, like RVs, is designed to provide temporary accommodation for recreation, camping, or seasonal use. Many local governments regulate PMUs in the same manner that they do RVs. 

Modular Housing

Modular housing is factory-built housing that is transported and assembled onsite. Modular construction, also known as off-site construction, has historically been used for single-family housing. More recently, it has also been used to construct multi-story commercial buildings (especially hotels) and multi-family housing.

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Under optimal conditions, there are a variety of benefits to factory-built modular housing, including:

  • Reduction of costs by shortening construction timelines, leading to faster “time to market” (versus traditional construction);
  • Efficiencies resulting in a lower sq. ft./unit cost and reduce project costs;
  • Safer working conditions, as compared to traditional, on-site construction;
  • Year-round modular housing factory operability in contrast to traditional construction's seasonal weather limitations;
  • Modular housing factories' cost efficiency due to streamlined assembly, better quality control, and more efficient use of materials, leading to leading to less waste, which also makes it ran environmentally sustainable approach; and
  • Modular housing's ability to be built simultaneously during the site preparation phase of construction (i.e., instillation of utilities, grading, building the foundation).

Since modular housing is a relatively new construction method, it is not widely used in Washington. This is due to several factors, mostly involving limited knowledge and experience. This general lack of understanding about modular construction extends from developers and contractors to government staff and lending institutions.

Recommended Resources for Modular Housing and Construction

Mass Timber

Mass timber is a group of framing styles typically categorized by use of large wood panels for wall, floor, and roof construction. This category includes multiple products, including cross-laminated timber, nail-laminated timber, glued-laminated timber, dowel-laminated timber, structural composite timber, and wood-concrete composites. Mass timber construction offers a lighter carbon footprint, making it a greener option than traditional construction methods, as well as being an efficient and safe method for use with large construction projects. It also offers cost savings over traditional construction methods for medium to high-rise buildings by using wood framing instead of more expensive concrete and steel framing.

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Photo courtesy of LEVER Architecture.

The Washington State Building Code Council made changes to allow for tall wood and mass timber construction in July of 2019, opening the door for mass timber construction projects (for example, see projects from TimberRise).

The Heartwood apartment building Seattle is an example of integrating mass timber design into an affordable housing development (with 126 affordable units), as well as sourcing wood products from regional supply chains to demonstrate the carbon sequestration benefits of timber. Life-cycle analysis reports highlight the project’s environmental and financial benefits at the end of its life, including reduced demolition costs and the potential for timber reuse, which can help offset some of the initial construction expenses associated with mass timber.

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Photo courtesy of LEVER Architecture.

Recommended Resources for Mass Timber


Last Modified: January 02, 2026