this is part of the article collection Village Buildings.
"A pattern language is a method of describing good design practices or patterns of useful organization within a field of expertise. The term was coined by architect Christopher Alexander and popularized by his 1977 book A Pattern Language." (-Wikipedia). Patterns "are in essence a way of capturing useful knowledge about the nature of a design problem, and expressing it in a way that can be easily shared and adapted to new contexts." (Mehaffy 2019). Alexander et al's 'patterns' concept has been widely applied in software development and other fields, and inspired the invention of the wiki, by Portland programmer Ward Cunningham for the Portland Patterns Repository.
Here we are attempting to derive a pattern language to map all possible ways to make housing affordable. Affordability is of course not the only problem or goal people have in housing or housing policy; people also seek housing quality, good design, housing that helps create community or builds wealth, or which helps employment and economic growth in an area, or which contributes to environmental sustainability, etc. Even closer to 'affordability,' one might choose different lenses such as anti-displacement, housing security, housing for all, adequate housing, attainable housing. Nevertheless, affordability (or the somewhat ambigious "affordable housing") is a pervasive concern and framing, and what we choose to focus on here.
Since affordability is defined by ratio of housing costs (perhaps including transportation) to income or wealth, of course a major way to help affordability is to increase the income/wealth of the less well-off. However, since this extends far into the realm of general economic/social policy, here we consider that outside of scope; except for the pattern of housing benefits (e.g. vouchers), which could and some argue should be done with general income benefits such as Earned Income Tax Credit. [this would tend to support people making their own determination of housing type/price that serves their needs, which is aligned with many of other patterns].
This affordability pattern language could be integrated with or extended to other pattern languages that describe other issues in housing or cities. For exampple, Wiki inventor Ward Cunningham, and fellow Portlander, urbanist, & architectural theorist Michael Mehaffy have been working with others on a new pattern language and online pattern repository, A Pattern Language for Growing Regions (APLGR; draft version online, printed book forthcoming 2019). The affordability pattern language here could potentially be integrated with APLGR, and others pattern languages such as a proposed "Portland Civic Patterns Repository"  to describe approaches for civic governance and engagement.
Mehaffy talks about wikis and pattern languages as tools for "consensus development." . In that vein, one of the purposes of this affordability pattern language is to suggest that quite varied patterns -- from public housing to 'abundant' market housing -- may all be sources of or factors in affordability, and considered practically rather than ideologically, they might often be combined in various ways, in a project or an environment.
Housing Affordability Patterns
(including building regulations reform)
Historically, many key improvements in affordability have arisen due to new transportation methods opening up new areas for residential development. For example: horse-drawn and then electric streetcars, subways / urban and commuter rail transit, and mass automobile ownership and freeways. Mobility improvement may be increased not only by the advent of major new transport technologies, but by more incremental improvements such as Bus Rapid Transit, or transit-oriented right-of-way design, or policies to subsidize transit.
Financial reform & innovation 
Tax preferences/incentives for affordable & non-profit housing.
Social Impact Bonds.
Cooperative financing/development. (cf German regulations facilitating).
Private for-profit or non-profit financing methods, e.g. currently there is a wave of ventures offering forms of shared equity or financial partnership to assist homebuyers.
Non-profit & Public housing; use of public land
[can be supported by bond and General funding - local/state. i.e., just tax ourselves more. (the "public option")].
Vouchers & direct subsidy: local, state, and/or Federal 
a 'choice' strategy.
[can be supported by bond and General funding - local/state. i.e., just tax ourselves more. (the "public option")]
public or private developed, e.g. Portland's Vanport, Headwaters Apartments/Village, Aloha Park Apartments (and other 1960s/70s Section 238 developments).
Rent regulation 
Oregon passes nation's 1st statewide rent regulation in 2019.
Inclusionary housing 
mandated, incented, or purchased. Example of Pearl District.
Lot division & 'condoization' 
Portland lot-division / narrow lots program.
Eli Spevak / Orange Splot projects; Pocket Neighborhoods, Villages: Ross Chapin, Cully Grove, SquareOne.
Sharing / Congregate housing 
SRO, co-housing, co-living.
e.g. Emerald Village, Eugene.
SquareOne Villages CLT-LEC model (Community Land Trust, Limited Equity Cooperative).
Self-, startup-, or public-financed. "Backdoor revolution."
Cully Land Trust, Emerald Village
Privately-developed affordable housing
built w/out subsidy. Guerrilla Development, Rob Justus / Home First.
Interim, mobile, or redeployable housing
POD, PAD Initiatives, Opportunity Village, OR Tiny House Code.
Alternative design & technologies
modular, manufactured, pre-fab, etc.
Informal, self-build, & incremental housing
Abundant, or "Naturally occurring affordable" housing
Housing for all.
Strategic Economics "Housing Tools & Strategies" Report 2019
Strategic Economics. "Eugene Housing Tools & Strategies Evaluation." [Prepared for City of Eugene, OR]. January 2019. https://www.eugene-or.gov/DocumentCenter/View/44616/Strategic-Economics-Final-Report-2019/.
--Zoning Strategies for Market Rate Housing Production
Streamlined Permitting Process
Accessory Dwelling Unit Ordinance
Graduated Density Zoning
Tweak traditional zoning code to encourage Missing Middle development
Mixed Use Zoning
Reduced or Flexible Minimum Parking Requirements
--Tax/Fee Exemptions and Other Incentives
Reduce impact fees/ systems development charges
Delay impact fees/systems development charges
Property tax exemptions
--Affordable Housing Production Strategies
Density Bonus Ordinances
Affordable Housing Overlay Zone
Workforce Housing Initiatives
--Local Affordable Housing Funding Strategies
Reduced Fees or Waivers
General Fund Allocation
In-Lieu Fees - Inclusionary Zoning
Residential Impact Fee for Affordable Housing
Commercial Development Impact Fee / Commercial Linkage Fee
Residential Construction Excise Tax
Commercial/ Industrial Construction Excise Tax
Affordable Housing Bond
--Preservation of Existing Affordable Housing Units
Condominium Conversion Controls and Tenant Protections
Mobilehome Park Conversion Controls and Tenant Protections
Single-Room Occupancy Conversion Controls / Preservation
Acquisition/ Preservation of DeedRestricted Units
Acquisition/ Preservation of "Naturally Occurring Affordable Housing"
Community Land Trust (CLT)
Short-term Rental Restrictions
--Tenant Protections and Support
Mobile Home Park Rent Control
Just Cause Eviction and Harassment Protections
Proactive Code Enforcement
Limit Discrimination Based on Source of Income
Homeowner Assistance Programs
Housing Rehabilitation Funds
Home sharing programs
Vacant Unit Tax
- Alexander, Christopher, and Murray Silverstein, Shlomo Angel, Sara Ishikawa, Denny Abrams (1977). A Pattern Language.
- Mehaffy, Michael W. (2019). A Pattern Language for Growing Regions [And Introducing An Online Repository of New Patterns]. Sustasis Press, 2019. [draft copy]. http://www.sustasis.net/APLFGR.html.
- Strategic Economics. "Eugene Housing Tools & Strategies Evaluation." [Prepared for City of Eugene, OR]. January 2019. https://www.eugene-or.gov/DocumentCenter/View/44616/Strategic-Economics-Final-Report-2019/.
- Wikipedia. "Pattern language." accessed 17 November, 2019. https://en.wikipedia.org/wiki/Pattern_language.