How to Create a Low-Level Design (LLD) Document
Once a High-Level Design (HLD) is approved, the next step is to prepare the Low-Level Design (LLD). While the HLD paints the big picture of the solution, the LLD dives into the technical details — it’s the manual that developers, engineers, and administrators will follow to actually build the system.
Think of the HLD as an architect’s sketch of a building, and the LLD as the engineer’s blueprint with exact measurements, wiring, and materials.
Why Do You Need an LLD?
An LLD ensures that:
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The design is implementation-ready with enough detail for engineers to execute.
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Every system component is described with configurations, dependencies, and integrations.
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Nothing is left to guesswork — reducing errors during build and deployment.
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Business and compliance requirements from the HLD are translated into technical specifications.
Key Sections of an LLD Document
Here’s a practical structure you can use when creating your own LLD:
1. Introduction and Scope
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Purpose: Why this LLD is being created.
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Scope: What’s included (in-scope) and what’s not (out-of-scope).
2. RAID (Risks, Assumptions, Issues, Dependencies)
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Capture technical risks (e.g., performance bottlenecks).
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List assumptions (e.g., required licenses will be available).
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Note issues and dependencies discovered at the detailed design stage.
3. Component Design
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Detailed description of each component (databases, apps, services, APIs).
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Implementation details for each component.
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Reference diagrams showing how components connect.
4. Platform Design
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Which infrastructure is used (IaaS, PaaS, SaaS).
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Compute/storage requirements and chosen SKUs.
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Deployment locations (cloud regions, data centers).
5. Network Design
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Network architecture with data flow diagrams.
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VLANs, IP address allocations, and firewall rules.
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Network zones (e.g., perimeter, client, services, DMZ).
6. Data Design
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Data governance and compliance considerations.
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How data is ingested, validated, and processed.
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APIs and interfaces used for data integration.
7. Application Design
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Modules and sub-modules included.
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Configuration details (parameters, APIs, dependencies).
8. Security Design
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Communication matrix (protocols, ports, source/destination).
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Certificates and authentication methods.
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User/group accounts and RBAC (role-based access control).
9. Operational Design
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Support model (1st, 2nd, 3rd line).
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Roles and responsibilities for managing the solution.
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Third-party vendor agreements (if applicable).
Tips for Writing a Good LLD
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Be specific – include configurations, IPs, SKUs, and protocols.
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Use diagrams – network flows and component maps help explain complexity.
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Document security clearly – communication rules, access control, and compliance are critical.
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Keep it consistent with the HLD – make sure every component in the HLD has a corresponding LLD detail.
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Version control – track changes properly, since LLDs evolve as systems are refined.
HLD vs LLD – The Difference
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HLD (High-Level Design) = What the system will look like (big picture).
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LLD (Low-Level Design) = How the system will be built (technical detail).
Together, they ensure that a project moves smoothly from concept to implementation.
Conclusion
The Low-Level Design is the bridge between architecture and execution. It leaves no ambiguity for engineers, ensures compliance with standards, and documents every technical choice. A strong LLD, when paired with a solid HLD, sets the foundation for reliable and scalable solutions.