System Design vs Low-Level Design Interview Differences

In technical interviews, design rounds typically come in two flavors: System Design (often called high-level design) and Low-Level Design (LLD, often focused on object-oriented design).

Both evaluate a candidate's design skills, but they differ significantly in scope and detail. Below, we break down the conceptual differences, interview expectations, and real-world examples for each to help you prepare effectively.

Conceptual Differences

System Design (High-Level Design) – System design deals with the architecture of an entire system or application. It provides a big-picture blueprint of how different components work together. This includes defining major modules or services, how they interact (APIs, communication patterns), and the technologies or frameworks involved. System design is about what the system should do and how it should be structured at a high level. Learn more about high level system design.

Low-Level Design (LLD) – Low-level design dives into the internal implementation details of a specific component or module. It outlines the classes, methods, data structures, and algorithms needed to fulfill the system’s requirements on a module-by-module basis. LLD is concerned with how the system’s features will be executed in code, often using object-oriented principles and design patterns. Learn more about low-level system design.

Key differences between system design and low-level design include:

Scope & Abstraction: System design covers the overall system architecture and module interactions, treating the system as a whole. Low-level design focuses on the internal workings of individual modules or components. In other words, system design is broader and more abstract, whereas LLD is narrower and more concrete in detail.
Focus: System design emphasizes what the system does – defining high-level components and their responsibilities. Low-level design concentrates on how the system achieves that functionality – the class designs, algorithms, and logic within each component.
Key Considerations: System design addresses non-functional requirements like scalability, reliability, and maintainability from an architectural perspective. Low-level design prioritizes code-level quality – ensuring maintainability, readability, and efficiency of the code, often through principles like modularity and reusability.
Output/Artifacts: In system design, you produce high-level diagrams or descriptions (e.g. architecture diagrams, service boundaries, data flow charts) to illustrate the system’s structure. In low-level design, the output could be class diagrams, pseudo-code, or actual code snippets showing how to implement the components’ logic.
Examples of Topics: System design covers components such as databases, load balancers, microservices, API design, caching systems, etc., to build a complete service. Low-level design deals with designing classes, relationships (inheritance/composition), methods, and using design patterns (e.g. Singleton, Factory) to solve a specific problem within a module.

Understanding these conceptual differences is crucial, as interviewers will expect you to approach each type of design question with a different mindset and level of detail.

System Design Interview Expectations

In a system design interview, the focus is on evaluating your ability to design a large-scale, high-level architecture for a given problem. Interviewers typically expect the following:

Requirement Analysis: You should start by clarifying the requirements of the system – both functional (features the system must have) and non-functional (scale, availability, latency, etc.). Identifying constraints and use cases upfront shows a structured approach.
High-Level Architecture: You will be expected to outline a robust architecture that meets the requirements. This means breaking the system into core components or services and explaining the responsibilities of each (for example, an authentication service, database, cache, load balancer, etc.). The design should cover how data flows between components and how they communicate (REST APIs, message queues, etc.). Trade-offs should be discussed – for instance, why you choose a NoSQL database over SQL, or how you balance consistency vs availability.
Scalability & Reliability: A key differentiator of system design is handling scale. Interviewers will probe how your design can accommodate growth or heavy load. You should discuss strategies like sharding databases, using load balancers, caching frequently used data, and adding replication for reliability. They will also expect consideration of fault tolerance (e.g. what happens if a service crashes) and high availability. In short, your architecture should be resilient and scalable by design.
Non-Functional Requirements: Beyond scalability, be prepared to address other systemic qualities. For example, explain how you’d ensure security (encryption, access control), performance (asynchronous processing, optimized data storage), and maintainability (clear module boundaries, using industry-standard tech) in your design. Interviewers often want to hear how you balance these concerns.
Communication & Justification: Throughout the discussion, clearly communicate your ideas. Use diagrams if possible (on a whiteboard or shared doc) to illustrate your design. Justify your choices by comparing alternatives. The interviewer is assessing not just what you design, but why — showing that you can reason about decisions and understand the implications. For instance, you might say, "I’ll use a CDN to serve static content to reduce latency for users globally," and mention why that's beneficial.
Breadth over Depth: Typically, system design rounds favor breadth of coverage of components over deep dive into one algorithm. You should touch on all major parts of the system at a high level. However, if the interviewer asks, you might need to zoom into a particular part (like the database schema or a specific interaction) briefly. The goal is to demonstrate you can handle the big picture and make it work as a cohesive system.

Remember, in system design interviews you usually do not write actual code. Instead, you present an architectural solution. The expectation is a well-thought-out architecture that could be handed to an engineering team to implement. As one guide notes, interviewers expect candidates to develop a high-level design covering all requirements and considering scalability, availability, and security in their decisions.

Learn more about functional vs non-functional requirements.

Low-Level Design Interview Expectations

A low-level design interview (also known as an Object-Oriented Design interview) assesses how you design the internal structure of a system or component in code. Here's what interviewers typically expect in LLD rounds:

Understanding of the Problem Domain: Just like system design, you should begin by understanding the requirements of the specific feature or component you need to design. For example, if asked to design a parking lot system, clarify how many types of vehicles, payment flow, etc., or if designing a class system for a chess game, clarify the rules to support. Understanding the use cases ensures your design will meet all functional requirements.
Class Design and Relationships: You will be expected to define the main classes (or objects) needed and their relationships. This involves choosing appropriate classes, attributes, and methods for each. A good approach is to identify the nouns (potential classes) and verbs (methods) from the problem description. For instance, designing a Library Management System might involve classes like Book, Member, Library, Loan, each with specific attributes and behaviors. Explain how these classes interact (e.g. a Member can borrow a Book via methods that update a Loan record). Interviewers will evaluate how you structure your classes and plan interactions between them – your design should be logical and cover the required functionality.
Object-Oriented Principles: Strong low-level designs demonstrate proper use of OOP principles such as encapsulation, inheritance, polymorphism, and abstraction. Interviewers often look for whether your design adheres to SOLID principles (for maintainable and extensible design). For example, is your code structured to allow easy changes (Open/Closed principle)? Did you avoid hard-coding specifics (Dependency Inversion)? If designing a game, perhaps you use inheritance (a ChessPiece base class with subclasses King, Queen, etc. to share common behavior). You should articulate these choices.
Use of Design Patterns: Applying relevant design patterns appropriately can set your solution apart. Interviewers may not require a specific pattern by name, but using well-known patterns to solve common design problems shows maturity. For instance, you might use a Factory Pattern to create different objects based on input, or a Strategy Pattern to define multiple behaviors. As an example, if designing an online payment system, you could use Strategy for different payment methods (CreditCard, PayPal, etc.). Demonstrating knowledge of patterns (factory, singleton, observer, etc.) to add features with minimal code changes is often seen as a positive. It shows you can create flexible and extensible designs.
Detail and Pseudocode: In LLD interviews, you often go deeper into certain methods or interactions. Interviewers might ask you to write pseudocode for a critical method or two to ensure you can implement the logic. For example, if you have a method addToCart(), they may ask how it would work – and you'd outline the steps in code form. In some cases, especially for lower experience levels, they might even expect a bit of actual code or a working prototype of a simplified version. As one experienced engineer noted, interviewers sometimes expect candidates to "magically present the best design with working code and test cases within 45-50 minutes". While writing a full, flawless code in a short interview isn't always realistic, you should be prepared to at least sketch out key algorithms or data structures for your design.
Code Quality Considerations: Beyond just making it work, explain how your design ensures code quality. This includes error handling, thread-safety (if relevant), and how you would test the components. For instance, if designing a multi-threaded producer/consumer system, mention thread safety in design. If designing classes for a feature, mention how you'd unit test each class or method. Interviewers appreciate when you consider edge cases and future maintainability (e.g., "I chose to separate the ParkingSpot class from Vehicle class so the code is easier to extend for different vehicle types without modifying existing logic – following open/closed principle"). Emphasize modularity and reusability – a good low-level design can be reused in different contexts and is easy to modify without breaking other parts. For example, a well-designed Notification class could be reused whether notifications are via email or SMS by swapping out a strategy object.

In summary, a low-level design interview expects you to produce a clean, extensible class design (often illustrated with a class diagram or by writing out class definitions) and possibly some pseudo-code for critical operations.

The discussion will be code-centric, albeit at a design level rather than writing full code.

Your goal is to convince the interviewer that you can translate requirements into a sound software design using proper coding practices.

Real-World Examples of HLD vs LLD

To better grasp the difference, let's look at how system design vs low-level design apply to real-world scenarios:

E-Commerce Platform (High-Level vs Low-Level): Imagine you're tasked with designing a new online shopping platform. In a system design context, you would outline high-level components: a service for user authentication, a product catalog service, an order processing service, a payment service, databases for storing product and order data, and so on. You'd describe how users interact with the system, how the services communicate (e.g. the order service calling the payment service), and how to scale each component independently. Now, zoom into one part of this system – say the Order Management module. The low-level design for that module would specify the exact data models and classes needed: perhaps classes like Order, ShoppingCart, Item, PaymentProcessor, each with their attributes and methods. You'd design methods like addToCart(item) or processOrder() with the logic for how an order moves from pending to confirmed, updating inventory, etc. The HLD gives a bird’s-eye view of the whole e-commerce system, while the LLD provides the nitty-gritty blueprint for one piece of it (order management) to be implemented in code. Learn how to design an e-commerce system.
Social Network or Messaging App: As another example, consider a real-time chat application. A system design interview for this might ask you to design something like WhatsApp or Slack. You would discuss servers for handling connections, databases for storing messages, load balancers to manage traffic, how to ensure messages are delivered in real-time, and strategies for scaling to millions of users (perhaps using distributed messaging queues, chat server clusters, etc.). In contrast, a low-level design question might be narrower, such as: "Design the data model and classes for a chat room feature." Your answer would involve classes like User, Message, ChatRoom, where ChatRoom contains a list of User participants and messages. You'd detail how a Message object is created and broadcast to users, how users join or leave a ChatRoom, and how to manage message history in code. The focus here is on the object interactions within a single chat room module, rather than the whole system's deployment architecture. Learn how to design Messenger.
URL Shortener Service: A popular system design example is "Design a URL shortener (like bit.ly)." At the system level, you'd describe components such as the web servers handling requests, an application layer to generate and map short URLs to long URLs, a database to store mappings, and considerations like caching frequently accessed URLs, handling redirects, and analytics tracking. For low-level design, you might instead be asked to implement the URL shortening logic in code. You would then focus on classes like URLShortener with methods encode(url) and decode(key), maybe a DatabaseInterface class to simulate storing and retrieving URLs, and discuss how to generate unique keys (using algorithms or libraries) and handle collisions. Again, the system design deals with infrastructure and high-level workflow, whereas the low-level design deals with specific code components and their logic. Learn how to design URL Shortener.

These examples illustrate that system design (HLD) is about seeing the forest – the entire system and its ecosystem – whereas low-level design (LLD) is about the trees – the detailed design of individual parts of the system.

In real-world projects, both levels are important: architects and senior engineers draft the high-level architecture, and developers implement modules based on low-level designs.

In interviews, you might be tested on one or both, depending on the role.

Conclusion

Both system design and low-level design interviews aim to evaluate your design thinking but at different levels of abstraction.

System design interviews assess how you architect a complete system, ensuring it meets broad requirements and can scale, while low-level design interviews assess how you craft the building blocks of the software, ensuring the code will be clean and maintainable.

Understanding these differences will help you tailor your approach in an interview: zoom out for system design to cover the big picture, and zoom in for low-level design to cover the details.

Practice both types of problems – sketch out architectures for big systems and draw out class diagrams for specific features.

By doing so, you’ll be well-prepared to discuss high-level trade-offs as well as in-depth object design.