SAP S/4HANA provides comprehensive core ERP functionality, but organisations may still need to adapt applications, processes and user experiences to meet specific operational requirements.
The challenge is to introduce these extensions without compromising the integrity of the core ERP system. SAP Build supports application development, process automation and digital workspaces through low-code, pro-code and generative AI capabilities.
However, not every extension should be developed in the same way. Depending on the requirement, organisations can use an on-stack extension, a side-by-side extension or a combination of both.
What is an on-stack extension?
On-stack extensions are developed and run directly within SAP S/4HANA.
Business experts can use this approach to make relatively straightforward changes, including:
- Modifying a user interface
- Adding or adjusting fields
- Introducing logic for a particular business process
Professional developers can also create more complex on-stack applications when detailed data requirements or frequent, high-volume access to SAP S/4HANA data are involved.
This approach can be appropriate when the requirement relates closely to the system’s underlying data or processes.
What is a side-by-side extension?
Side-by-side extensions are decoupled from the core ERP system, providing greater flexibility for requirements that extend beyond SAP S/4HANA.
They can support:
- Integration between SAP and third-party solutions
- Web and mobile applications
- Mobile and offline access
- Processes operating across different applications and platforms
- Information drawn from multiple business systems
This approach allows organisations to introduce new functionality while reducing its effect on the core system.
On-stack or side-by-side: which approach fits the requirement?
There is no single approach suitable for every SAP S/4HANA extension.
A relatively simple change within SAP S/4HANA may be suited to an on-stack extension. For example, an organisation might need to add a custom field, modify a user interface or introduce additional process logic.
A side-by-side approach may be more appropriate when an application needs to connect SAP S/4HANA with other SAP or third-party systems. It can also support applications requiring mobile or offline access and processes spanning several platforms.
Some requirements may involve both approaches. An organisation could make a change within SAP S/4HANA while also developing a separate application, bringing together information from multiple systems.
The appropriate choice depends on factors including:
- The business requirement
- The data involved
- The integrations required
- How frequently the application needs to access SAP S/4HANA data
- Where and how the resulting application or process will be used
These factors must be considered together. The choice is not simply between a basic and a more advanced extension, but between approaches designed to support different operational and technical requirements.
How the approaches can work across the business
On-stack and side-by-side extensions can support requirements across sales and service, supply chain, finance, operations and IT.
Sales and service
Sales and service professionals may need access to customer information while working away from their desks.
An on-stack extension could modify the SAP S/4HANA user interface or add fields for specific information. A side-by-side mobile application could consolidate customer data from SAP and third-party systems and make it available to employees wherever they are working, including in offline environments.
Supply chain
Supply chain extensions can help replace paper-based work orders, handwritten updates and delayed data entry.
For example, a mobile application could provide technicians with assembly instructions, photographs and progress-tracking capabilities. It could also support the ordering and approval of required parts.
On-stack logic could be used to reorder inventory proactively, while a side-by-side extension could connect several systems and provide a wider view of supply chain activity.
Read more about improving supply chain flexibility with SAP Build.
Finance
Finance teams can use extensions to reduce manual intervention in accounts receivable and accounts payable processes.
Vendor invoices, for example, can be identified, matched and processed automatically, with workflows supporting approvals and exception handling.
An on-stack extension could automate a process within SAP S/4HANA. A side-by-side extension could bring together information from SAP and third-party applications and automate processes across multiple business systems.
Operations
Operations teams need timely access to information about business performance.
SAP Build can support the development of a portal that tracks KPIs using SAP S/4HANA data. If performance begins to decline or is expected to miss a target, employees can contribute ideas, assess possible scenarios and implement a response.
An on-stack extension could add a field supporting a critical KPI. A side-by-side application could connect information from several systems and present actionable insights through web or mobile applications.
IT and application development
SAP Build allows both business experts and professional developers to contribute to SAP S/4HANA extensions.
Low-code tools provide drag-and-drop capabilities for designing applications, while professional developers can use pro-code tools and languages including ABAP, Java and JavaScript.
Generative AI can assist with application logic, data models, sample data, unit tests, user interfaces and back-end development. Joule can also support code generation and workflow design across pro-code and low-code tools.
Discover more about application development with SAP Build Code and Joule.
Maintaining a clean core while extending SAP S/4HANA
The need to extend SAP S/4HANA must be balanced with the need to protect the core ERP system.
Clean core principles help organisations introduce extensions and updates while reducing disruption to core functionality, protecting integration connections and retaining access to new software updates and technologies. Clean core also extends beyond application architecture to areas such as data governance, quality and protection.
On-stack and side-by-side extensions provide different ways to achieve this balance. In some cases, the requirement may be met by a focused change within SAP S/4HANA. In others, separating the extension from the core provides the flexibility required to connect systems, support mobile working or automate wider processes.
Understanding the requirement before selecting the approach is therefore essential.
On Device Solutions also offers an SAP-certified clean-core integration solution. ODS ABAP Integration Suite V5.0.0 has achieved SAP Integration Certification – Clean Core, validated for deployment on SAP S/4HANA Cloud Private Edition, release 2025. Learn more about the certification and ODS’s clean-core capabilities.
Selecting the right SAP S/4HANA extension approach
SAP Build gives organisations several ways to adapt SAP S/4HANA as business needs change. The central question is not simply what can be built, but where and how the extension should be developed.
The answer will depend on the organisation’s processes, data, integrations and intended use of the solution. Some requirements may call for an on-stack extension, others for a side-by-side application, and some for a carefully planned combination of both.
Considering how to extend SAP S/4HANA while maintaining a clean core?
Contact On Device Solutions to discuss your requirements.
