Since the launch of ABAP Cloud in late 2022, SAP has provided organizations with a comprehensive extensibility guide for custom code development in S/4HANA. This guide has been updated several times as the extensibility model evolved — most significantly in August 2025, when the 3-tier model was superseded by the Clean Core Level Concept. Here is what every ABAP architect and developer needs to know about the current extensibility framework.
From 3-Tier to Clean Core Levels: The Evolution
The original 3-tier extensibility model (introduced in 2022) organized ABAP custom development into three tiers: Tier 1 for ABAP Cloud on-stack and BTP side-by-side, Tier 2 for wrapper-based access to not-yet-released APIs, and Tier 3 for classic ABAP. While this was a major step forward, feedback from early adopters revealed practical limitations — particularly around the complexity of wrapping APIs in Tier 2 and the binary nature of the Tier 1/Tier 3 boundary.
The updated model introduces Clean Core Levels A through D, which provide a more granular and pragmatic classification. This change brings the official SAP guidance in line with how development teams actually work in complex enterprise landscapes.
Key Extensibility Options in S/4HANA Cloud
Key User Extensibility (In-App)
Key User Extensibility allows business users and configurators to extend SAP applications without any ABAP development. Custom fields can be added to existing SAP business objects using the Custom Fields and Logic app. These extensions are automatically upgrade-safe and can be transported between systems using the standard transport mechanisms.
Developer Extensibility (ABAP Cloud On-Stack)
Developer extensibility covers ABAP Cloud development directly in the S/4HANA system. This includes creating new business objects using RAP, implementing BAdIs (Business Add-Ins) for custom logic within SAP processes, and building new OData services. All development must use released ABAP APIs (C1-released objects) and comply with the ABAP for Cloud Development language version.
Side-by-Side Extensibility (SAP BTP)
Side-by-side extensions run on SAP BTP independently of the S/4HANA system, communicating via APIs (OData, IDoc, SOAP, or event-based). This pattern is ideal for complex integrations, process automation with SAP Build Process Automation, and custom apps built with SAP Build Apps or CAP (Cloud Application Programming model). Side-by-side extensions are the most decoupled option — they require no ABAP development and create zero upgrade risk to the core system.
The ABAP Cloud Development Model: Core Principles
The ABAP Cloud development model enforces clean core compliance through a combination of technical controls and governance guidelines:
- Released API contract: Only objects marked as C1-released (with @ObjectModel.lifecycle.contract.type: #PUBLIC_LOCAL_API or equivalent) may be used in ABAP Cloud code. SAP guarantees these interfaces are stable across upgrades.
- ABAP language version enforcement: Objects developed for cloud use must set the ABAP language version to “ABAP for Cloud Development,” which syntactically prohibits non-cloud-safe statements.
- Software component separation: Custom developments are placed in customer software components (Z* or Y* namespaces) that are separate from SAP delivery components, ensuring clear ownership and transportability.
- ATC enforcement: Automatic checks via the ABAP Test Cockpit catch violations before they reach production.
Practical Guidance: Choosing Your Extensibility Path
Selecting the right extensibility approach for a given requirement is one of the most consequential architectural decisions in an S/4HANA implementation. A useful decision framework:
Start with Key User Extensibility whenever possible — it requires no coding, is fully upgrade-safe, and can be delivered faster than any development option. If the business requirement exceeds what key user tools offer, consider ABAP Cloud on-stack developer extensibility using BAdIs or new RAP objects. Reserve side-by-side BTP extensions for scenarios that require decoupling, multi-system integration, or non-ABAP technologies. Use classic ABAP (Levels C/D) only where absolutely necessary, and document it explicitly in your clean core governance register.
The SAP ABAP Extensibility Guide is a living document — updated with each major S/4HANA release — and should be your primary reference for current best practices. Consulting it alongside the Clean Core Level Concept framework will ensure your extensibility strategy stays aligned with SAP’s evolving platform direction.