Low Level Architecture Design Document

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:

• The design is implementation-ready with enough detail for engineers to execute.

• Every system component is described with configurations, dependencies, and integrations.

• Nothing is left to guesswork — reducing errors during build and deployment.

• 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

• Purpose: Why this LLD is being created.

• Scope: What’s included (in-scope) and what’s not (out-of-scope).

2. RAID (Risks, Assumptions, Issues, Dependencies)

• Capture technical risks (e.g., performance bottlenecks).

• List assumptions (e.g., required licenses will be available).

• Note issues and dependencies discovered at the detailed design stage.

3. Component Design

• Detailed description of each component (databases, apps, services, APIs).

• Implementation details for each component.

• Reference diagrams showing how components connect.

4. Platform Design

• Which infrastructure is used (IaaS, PaaS, SaaS).

• Compute/storage requirements and chosen SKUs.

• Deployment locations (cloud regions, data centers).

5. Network Design

• Network architecture with data flow diagrams.

• VLANs, IP address allocations, and firewall rules.

• Network zones (e.g., perimeter, client, services, DMZ).

6. Data Design

• Data governance and compliance considerations.

• How data is ingested, validated, and processed.

• APIs and interfaces used for data integration.

7. Application Design

• Modules and sub-modules included.

• Configuration details (parameters, APIs, dependencies).

8. Security Design

• Communication matrix (protocols, ports, source/destination).

• Certificates and authentication methods.

• User/group accounts and RBAC (role-based access control).

9. Operational Design

• Support model (1st, 2nd, 3rd line).

• Roles and responsibilities for managing the solution.

• Third-party vendor agreements (if applicable).

Tips for Writing a Good LLD

• Be specific – include configurations, IPs, SKUs, and protocols.

• Use diagrams – network flows and component maps help explain complexity.

• Document security clearly – communication rules, access control, and compliance are critical.

• Keep it consistent with the HLD – make sure every component in the HLD has a corresponding LLD detail.

• Version control – track changes properly, since LLDs evolve as systems are refined.

HLD vs LLD – The Difference

• HLD (High-Level Design) = What the system will look like (big picture).

• 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.