A guide to software design documentation and specifications

What is a software design specification?

A software design specification, or software design document (SDD), is a comprehensive document that outlines a software development project's architecture, components, interfaces and other crucial elements.

At its core, an SDD is a blueprint that guides developers, designers and stakeholders through the software development process. It includes detailed descriptions of the system's functionalities, data structures, algorithms and UIs. By defining these elements upfront, the SDD helps ensure all team members understand the project's goals and requirements. This clarity is essential for maintaining consistency and quality throughout the DevOps lifecycle, as it reduces the likelihood of miscommunications and design flaws.

Why are software design specifications important?

The importance of software design specifications cannot be overstated. First and foremost, they act as a contract between stakeholders and developers, ensuring that all parties are aligned on the expected outcomes. This alignment minimizes the risk of scope creep and helps keep the project on track and within budget. Moreover, an SDD provides a reference point that can be used for future iterations, maintenance and upgrades, facilitating more efficient modifications.

A well-documented design specification also lets teams onboard new members more quickly, as the document serves as a comprehensive guide to the system's architecture and functionality. Ultimately, investing time in a quality SDD pays off by fostering better collaboration, improving code quality and enhancing the overall success of the software project.

While the table of contents of SDDs may vary across the tech industry, the benefits remain the same:

• Serves as a single source of truth. The SDD defines the project goals, scope, business and technical requirements in detail and includes related information to ensure no misunderstandings among the development team, product management and executive stakeholders.
• Avoids scope creep. Software design documents aid project managers in accurately estimating the resources, time and budget required for the project by outlining the detailed design and technical requirements. Project teams can also use detailed software design documents to avoid scope creep and identify potential risks and mitigation strategies early.
• Facilitates collaboration. Publishing the SDD to a team collaboration tool, such as a Notion wiki or Confluence space, facilitates teamwork and coordination, especially in cross-functional teams. Each developer knows their role and responsibilities, reducing overlaps and ensuring that all components are developed in harmony. This is particularly important in Agile environments where iterative development and continuous integration require efficient team coordination.
• Fosters quality. The SDD is a quality benchmark for the development project because it ensures that the final product meets the required design and functionality. It also sets the criteria for success and standards for coding practices, testing procedures and documentation.

How to write a software design specification

Writing a software design document requires some preparation since the document needs the expertise and input of a cross-functional team, whose members have other responsibilities than just writing design documents.

A typical team for authoring design specifications includes the following:

Writing a software design document requires some preparation since the document needs the expertise and input of a cross-functional team, whose members have other responsibilities than just writing design documents.

• Developers.
• Enterprise, cloud and solution architects.
• Testers.
• Business analysts.
• UI and UX designers.
• Technical writers.

Once the cross-functional team is assembled, there are several key preparatory steps the team must take to ensure successful document development:

1. Define SDD roles and responsibilities. It's important to clearly define SDD roles and responsibilities to streamline document development. Consider the availability of senior developers and architects for writing responsibilities. Provide technical writing support to these developers, and make an extra effort to focus them on SDD sections where their expertise benefits most.
2. Gather requirements. Writing an SDD starts with interviews to gather stakeholders' requirements and expectations. Existing requirements documents, user stories and related documentation are initially collected and reviewed by the team at this time.
3. Map system integrations. Another important preparatory step is to create diagrams to outline the system's environment and interactions with other entities, such as other back-end systems in the organization. The document should also capture how the system interacts with partner or vendor systems, such as third-party payment processors. Take the extra time to develop use cases that capture functional requirements and convey how different system parts interact.
4. Define project objectives and scope. Define clear objectives and scope for the project to ensure alignment with the organization's business goals. Now is the time to involve business stakeholders or their designees in the discussion. Get them on a Zoom call or, better yet, in the same room with the project team to collaborate, debate and ask questions of one another.

Contents of a software design document

There's more than one way to create an SDD template. Ultimately, the writer must collaborate with development teams and stakeholders to establish a software design document standard for the organization. Some standards, such as IEEE 1016-2009, govern the information content of SDDs. However, many organizations work with a customized SDD that suits their needs.

Here are some common elements of an SDD for reference.

1. Front matter

The front matter of an SDD includes the following:

• Project title.
• Project owner.
• Authors.
• Development team members.
• Version history.

Typically, SDD front matter also includes a version table that documents the versioning and changes the document undergoes. While a review table may seem like a relic of the past, compliance auditors look for this information as part of their audits.

2. Introduction

The introduction of an SDD includes a short five- to 10-sentence abstract introducing the project. It also includes a glossary of terms in the SDD and lists the project goals. When writing an introduction, consider audiences who aren't developers, such as the executive team, who may need a quick introduction to the project.

3. Current design

This is where the team includes the current design of the application. The team might be reverse-engineering an application's documentation, such as writing an SDD for an application already in production with nonexistent or significantly outdated documentation. There are also cases when documenting the current design is helpful, such as in migrating a legacy on-premises application to cloud services.

4. Proposed design

The proposed design section of a software design document includes a detailed description of the new system's architecture, functionality and components.

This is usually one of the largest sections of an SDD, as it outlines the design patterns and principles for implementing the software. This section also provides diagrams and models to represent the system's structure and data flow visually. Additionally, it explains how the proposed design meets the project requirements and addresses any identified problems. Finally, it includes a discussion of the chosen technologies and tools and their justification for use in the project.

5. Planned changes

Include a detailed list of planned changes for the project. The best way to communicate such changes is in a table format that details the following:

• Overview of changes, including the purpose of changes and their expected impact.
• Detailed description, including current state, proposed state and justification.

The scope of changes outlines which application components are affected by dependencies between the planned changes and other system components or external systems.

6. Test plan

The test plan section of the SDD should include a link to the project's test plan. While it was once standard practice to include a complete test plan, the current era of online documents makes it easier to just link to the test plan.

7. Monitoring plan

This section includes a detailed monitoring plan, including relevant observability metrics once the application receives user traffic during the pilot phase.

8. Deployment plan

The deployment plan section of the SDD includes a detailed plan and framework for deploying the application into production. The deployment plan is one of the largest sections of an SDD, covering several aspects of software deployment:

• Introduction, including a brief overview of the deployment plan, deployment objectives and key stakeholders.
• Deployment strategy, including deployment approach and strategy justification.
• Predeployment requirements, including system and environment prerequisites, configuration settings and data migration requirements, as well as security and compliance checks.
• Deployment phases, which include detailed steps for each phase of the deployment, timeline, milestones and team member responsibilities.
• Deployment environment, especially the target environment(s).
• Environment setup and configuration steps.
• Verification and validation procedures.
• Tools and resources, including a list of tools required for deployment and resource allocation.
• Deployment testing, including predeployment testing procedures, successful deployment criteria and postdeployment validation steps.
• Risk management, including identification of potential risks, mitigation strategies and contingency plans.

9. Communication plan

The communication plan section of a software design document outlines the stakeholder communication strategy, detailing how and when to inform stakeholders about project progress and key decisions. It specifies the deployment schedule and notifications, ensuring that all relevant parties know the timelines and any potential system downtimes.

This section also describes the reporting process, including the frequency and format of progress reports to keep everyone aligned. Additionally, it details the documentation process, highlighting what documents will be created, maintained and shared throughout the project lifecycle. The plan ensures that communication channels are clearly defined and accessible to all stakeholders. Finally, it includes a feedback mechanism to effectively gather and address stakeholder concerns.

10. Documentation plan

It's essential to treat technical documentation as part of the product, so include a section about the technical documentation that will be shipped with the product. Detail the strategy and steps behind creating online help and user assistance.

11. Rollback plan

Rolling back to an earlier version of the application requires planning and orchestration, primarily when the application serves external, paying customers. Include a detailed plan in the SDD that maps out the rollback plan for the project step by step.

12. Project impact

The project impact section of the SDD should focus on cost analysis, security analysis, implications for other business units and risk analysis.

13. Evaluation criteria

The evaluation criteria section should include specific metrics and benchmarks to measure the system's performance and effectiveness. It should outline the testing procedures and success criteria for functional and nonfunctional requirements, such as usability, reliability and scalability. Additionally, this section should specify the methods for gathering and analyzing user feedback to ensure the design meets the stakeholders' needs and expectations.

14. Work timeline

Software design documents should also communicate the work timeline for the project. If publishing the software design document to a team collaboration tool, such as a Confluence space or Notion wiki, look for opportunities to link it to the project schedule in the project management application or embed the timeline into the SDD.

15. References used

This section is for capturing and documenting any third-party resources that contributed to the stages of document development. Some standard references to include are the following:

• Internal documents, such as requirements documents and related.
• Analyst reports that were consulted during SDD writing.
• Internal strategy documents and slide decks.

Examples of software design specifications

It's commonplace to hold onto templates and documents from past employers and contracts in the IT industry. Think of them as bring-your-own templates. SDD templates vary so much from organization to organization because of the needs of customers, stakeholders, business contracts and other related matters. There's nothing wrong with working from an externally sourced template if writers treat it as a starting block and still get feedback from their teams and stakeholders about what they require in a standard SDD format.

Use these SDD templates to guide the document development process.

The "Design & System requirements template" is available in the Notion template library. It was created by Odette Jansen.

Nuclino is a SaaS workspace product that includes a product specification template.

Tips for writing a good software design document
Writing an SDD is often a team effort, bringing together the expertise and talents of a team. Not everybody may be a writer by trade.

Managing content

An important part of writing an SDD is managing and organizing the writing team according to the team members' expertise.

• Set expectations that not everybody on the team must work on the entire SDD, and make assignments accordingly.
• Appoint a document owner, who could be a business analyst, technical writer or technical leader, to coordinate the writing and editing of the SDD.
• Assign a core team on the SDD project from ideation to completion with enough technical knowledge to combine content from different authors to be in one voice and style.
• Avoid holding any engineers or technical staff to a corporate style guide to dictate their writing style. Rather, set them up for success by encouraging them to keep their writing simple and concise.
• Partner team members who aren't comfortable with writing with someone who is. Technical writers or editors can provide them with actionable feedback during the writing phase.

Editing an SDD

It's essential to be granular in editing an SDD because of the nature of short timelines and cross-functional teams. Focus on the team's expertise when editing an SDD. For example, don't ask architects or vice presidents to edit for grammar because the result might be an SDD with more grammatical mistakes than the original document. Define editing roles based on team members' strong suits, such as the following:

• Development editing. This is ideal for business stakeholders to ask questions about the draft. They can focus on the content and deliver constructive feedback on the direction of the SDD.
• Technical reviewing. Engineers and architects review the SDD for technical accuracy and feasibility.
• Content editing. The technical writer reviews the SDD content for grammar, flow, consistency and impact. Typically, their edits may raise questions for the team's analysts, engineers and architects.

Since SDD documents draw input from multiple authors, tracking edits and comments that arise during the editing phase is important. If the team works in Microsoft 365 for edits, use track changes and comments in Microsoft Word. Likewise, use suggestions and comments in Google Docs if the organization uses Google Workspace.

Writing an SDD

A complex and poorly written SDD is a project management accident waiting to happen. Even if there is a technical writer assigned to the SDD project, follow these essential tips:

• Write in short sentences and short paragraphs for readability.
• Be generous with examples, especially bills of materials and diagrams where appropriate.
• Implement an AI writing assistant, such as Grammarly for Business, to help SDD writers.
• Make generous use of heading styles to make the SDD easier to read.

Using generative AI to write drafts can be risky, especially if the organization doesn't have a general AI usage policy. Any data team members enter into public generative AI tools, such as ChatGPT, could be used for training data, which means SDD content might appear in the public domain.

Including visuals

Using visuals in an SDD improves readability. Visualization tools have come a long way from Microsoft Visio on the desktop, so consider other options, such as embedding Miro diagrams in the document. Miro is a team collaboration and project management platform.

Here are some visuals to consider for the SDD:

• Cloud architecture diagrams.
• Network diagrams.
• Timelines.

Seeking feedback

Seeking feedback on the SDD should be a continuous act, not a one-time review. The team and internal stakeholders should provide input on every significant version of the document.

Regularly updating the document

Developing the SDD for a complex cloud-native application requires time and expertise, which teams can recoup by implementing processes and gates to ensure that the development teams update their software design documents as part of the project lifecycle.

Will Kelly is a freelance writer and content strategist who has written about cloud, DevOps, AI and enterprise mobility.