The RESO Data Dictionary is the Real Estate Standards Organization’s field-level vocabulary for property data — the shared nouns of American residential real estate. It defines resources including Property, Member, Office, Media and OpenHouse, along with their fields, enumerations and lookups. It is the semantic half of the RESO standard, separable from the RESO Web API that transports it, and it is the half that most industries never manage to agree on.
RESO Data Dictionary
The RESO Data Dictionary is the field-level vocabulary underneath American residential real estate. Where the RESO Web API defines how listing data moves, the Data Dictionary defines what the fields mean — Property, Member, Office, Media and OpenHouse, with their enumerations and lookup resources. Versions 1.7, 2.0 and 2.1 are all in active use across the market simultaneously.
- The semantic half - Transport is the easy part. A shared, versioned vocabulary that hundreds of independent cooperatives actually agree on is the rare achievement, and this is one of the few working examples in any vertical.
- Mandated, not merely published - National Association of REALTORS Policy Statement 7.90 requires association-owned multiple listing services to certify against it within a year of each ratification. That mandate comes from an industry body rather than a regulator — nearly unique in the API economy.
- Certified - RESO runs a public conformance directory. Trestle and MLS Grid both hold Data Dictionary 2.0 certifications, and the certificates are verifiable without an account.
- Free to read - The specifications are anonymous downloads on GitHub and reso.org. Reading the standard costs nothing; calling an endpoint that implements it is another matter entirely.
The finding that earns this entry its own page comes from The State of US Real Estate APIs: across
forty-five US real estate organizations, not one publishes a machine-readable specification for
mls, idx, media, openhouse or member — the Data Dictionary’s own core resources. Only three
publish anything with “listing” in the filename, and one of those is a portal subsidiary. What the
industry does publish is generic CRM plumbing: contacts, deals, tasks, documents, users, webhooks.
That is the sharpest available illustration of a distinction worth carrying into any standards work. A vocabulary can be well-designed, freely published, formally certified and genuinely mandated, and still never surface as a public contract — because none of those things is the same as deciding who may call the endpoint. The Data Dictionary succeeded completely at agreement and was never asked to do anything about access.
Referenced in API Evangelist papers
This standard shows up in my published research. These reports read the machine-readable evidence provider by provider — and put this standard in the context of a real sector.
The State of US Real Estate APIs
Mandated by NAR for association-owned MLSs — and across 45 US real estate organizations, ZERO publish a specification for its core resources: mls, idx, media, openhouse or member.
The State of Canadian Real Estate APIs
CREA adopted a RESO-shaped Web API for its Data Distribution Facility voluntarily, with no Canadian body mandating it — evidence that vocabularies spread on commercial need, not compulsion.
Governed by these regulations
A standard is the machine-readable contract; a regulation is the law that requires it. These are the regulations that mandate or drive this standard, catalogued at regulations.apievangelist.com.
NAR MLS Policy Statement 7.90
Mandates certification against this vocabulary — whose core resources no US organization publishes a spec for.
