RESO Data Dictionary 2.1 is here: What it means for MLSs still running on DD 1.7

As real estate technology accelerates, data consistency has evolved from a back-office convenience into the bedrock of modern property intelligence and brokerage operations. Brokerages demand faster feeds, consumer portals require higher-fidelity metadata, and agents rely on integrated tools that simply cannot afford schema mismatches.

Against this backdrop, the Real Estate Standards Organization (RESO) continues to advance its core standards, with Data Dictionary 2.1 building directly upon the foundational leaps of the previous version.

For industry leaders, however, the arrival of Data Dictionary 2.1 highlights a growing divide. Roughly 40% of certified MLSs are currently at risk of losing their RESO certification within the next six months. Even more concerning, over 125 MLSs across the United States have never received a RESO certification at all.

If your organization is still operating on Data Dictionary 1.7—or navigating proprietary legacy feeds—the window to modernize is closing fast. Here is a clear breakdown of what Data Dictionary 2.1 introduces, why legacy versions can no longer keep up, and how you can achieve compliance without exhausting internal resources.

The growing divide: Why DD 1.7 is no longer enough

For years, Data Dictionary 1.7 served as a reliable industry benchmark. But the real estate landscape has outgrown it:

Rigorous certification & validation
Data Dictionary 1.7 largely trusted that a vendor's data matched what they advertised – and complaints abounded. Data Dictionary 2.0+ enforces strict schema validation and compliance testing, ensuring actual data matches the standard.

New and expanded fields
The new version adds important modern real estate data points—such as CurrentPrice, expanded multi-family and building-level attributes, OfficeNationalAssociationId, AttributionContact, PetsAllowed, and updated water/cooling/security lookups. RESO has made eight years of progress on MLS and broker requests since version 1.7 and it’s up to MLSs to implement those standards, rather than being the standards bottleneck.

Data type fixes
The new version converts several legacy Int64 fields (such as agent and building identifiers) into consistent String types to prevent integration bugs.  

Better interoperability
Software and consumer-facing tools experience reduced time, cost, and friction when onboarding new MLS feeds because the pick-lists and attributes are tightly synchronized. New versions of the Data Dictionary help new technology efficiently come to market for the MLS and its subscribers.

Metadata and enumeration gaps
Data Dictionary 1.7 leaves too much room for custom mapping workarounds and ambiguous enumerations. Downstream data consumers frequently have to write costly, custom translation layers just to interpret basic property details.

Legacy designations and deprecations
RESO has moved older versions into "Certified Legacy" territory. Systems running legacy schemas will increasingly struggle to pass automated testing or interface with newly developed broker tools.

The 6-month expiration cliff
With four in ten certified MLSs facing certification expiration in the coming months, letting renewals slide means risking member service disruptions, broker dissatisfaction, and non-compliance with national industry mandates.

Simply put: Data Dictionary 1.7 is no longer good enough to power the next generation of real estate applications.

What changes with Data Dictionary 2.1?

Data Dictionary 2.1 is designed to refine consistency, eliminate naming ambiguities, and provide deeper support for modern workflow integrations. Key structural evolutions include:

Singular resource naming conventions
To align with standard API practices and improve consistency across data models, multi-entity resources transition from pluralized to singular formats (such as Contact, Team, PropertyRoom, PropertyUnitType, and Rule). This standardizes object modeling across client applications and reduces query syntax friction.

Targeted payload additions  
Data Dictionary 2.1 introduces payload tags tailored specifically for Association Management Systems (AMS) and Offer Management platforms. This allows systems to ingest and display transactional data efficiently without having to parse massive, monolithic listing payloads.

High-precision timestamps and event tracking
Modern transactions require exact event tracking. Additions such as dedicated modification timestamps for open houses and listing statuses (e.g., tracking when a property returns to market) ensure consumer displays and agent alerts reflect real-time property availability without lag or guesswork.

Deeper support for commercial and expanded property data
From expanded Commercial Real Estate (CRE) categorization and tiered property hierarchies to specialized fields accommodating evolving residential attributes, DD 2.1 builds a bridge for data that historically lived outside standard listing fields.

The modernization bottleneck for MLS teams

If the advantages of DD 2.1 are so compelling, why are over 125 MLSs uncertified, and why are hundreds more staring down expiration deadlines? The answer comes down to bandwidth and technical friction.

Strained internal engineering
Most MLS technical teams already carry a full queue managing daily help desk requests, identity management, and platform migrations. Re-architecting legacy database schemas and re-mapping hundreds of custom fields can feel like an impossible undertaking.

Fear of breaking downstream integrations
Upgrading major data schemas can raise concerns about disrupting local third-party vendor connections, IDX feeds, or custom member tools.

The "fine print" trap
Some technology vendors have recently begun advertising "free" certification programs, only to reveal restrictive qualification requirements, multi-year lock-in agreements, or exclusivity clauses hidden in the fine print.

Standards should elevate the entire industry, not divide it into organizations that can afford specialized integration consultants and those left behind on outdated schemas.

Universal standards shouldn’t come with asterisks

At Cotality, we believe that open, interoperable real estate standards must be accessible to every market participant—regardless of size or budget.

That is why we are certifying any MLS free of charge. Large or small, coast to coast.

We don’t believe in qualifying questionnaires, hidden charges, or exclusive clubs. We provide zero-cost certification because Cotality has built the dedicated infrastructure, institutional backing, and architectural scale necessary to streamline data compliance across the real estate ecosystem.

When your MLS achieves modern RESO certification, local brokers get cleaner feeds, technology vendors integrate faster, and your internal staff is freed from continuous schema troubleshooting.

How to future-proof your MLS for 2027

Upgrading from Data Dictionary 1.7 or certifying for the very first time does not require months of disruption.

Step 1: Status audit. We evaluate your current schema against current RESO benchmarks to identify legacy fields and missing enumerations.

Step 2: Automated heavy lifting. Cotality’s platform maps your local data structures directly into the modern RESO specification, preserving your unique local attributes while achieving standardized compliance.

Step 3: Frictionless verification. We run the testing protocol directly through the RESO Analytics engine to secure your active certification.

Ready to rise to the new standard?

Whether your certification is expiring in the next six months or your organization needs help getting certified for the first time, our team is ready to step in.

Connect with the Cotality team today to claim your free RESO certification and get your MLS fully compliant for 2027.

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