What is software design documentation (SDD)?

A software design document (SDD), or software design specification, is a detailed software development plan. It includes specifications that define the software’s functionality and possibly project management aspects such as timelines and goals.

Independent contractors and project managers at large companies use SDDs, before starting development, usually for projects that take longer than a month. Skipping these planning steps to start coding is inadvisable, as a lack of information can cause issues during later phases. Smaller teams typically use shorter SDDs.

SDDs help explain many important aspects of a software development project, including the problem the software attempts to solve, how the final product should look, and how the internal architecture functions.

This is part of a series of articles about software documentation.

Why are software design documents important?

Here are the benefits of software design documentation:

  • Standardization—the SDD ensures all stakeholders involved in the software development project are aligned on the principles and design of the system that will be designed.
  • Coordination—the SDD helps coordinate the efforts of large teams, providing a clear reference point and information on all software parts and how they should operate.
  • Requirements—software development teams use an SDD to ensure they build a product that meets the project’s requirements according to prior agreement.
  • Planning—team leaders use the SDD to plan the entire system architecture and prepare for possible challenges, exposing possible gaps.

What should be included in software design documents?

User interface

The user interface (UI) is among the most challenging aspects of a design document. Several types of roles work on the UI, each using different skillets. As a result, communication issues might occur. Including the UI in the documentation helps clarify expectations and establish clear communications between all involved parties.

Typically, a graphic designer creates illustrations of the UI, which the product owner sends to the developers. However, development teams communicate in technical terms, while design teams typically consider the UX and visual aspects of the UI, leading to communication issues.

Creating illustrations helps, but a good SDD should include additional context to clarify other aspects, such as control states and animations. It should also define how the end-user should navigate the application and what is the UI flow for important user stories..


A good SDD must define the project’s functionality, including various aspects, such as what the application should do and how quickly it should perform its functions. It is important to clarify which operations are required at the first execution and outline requirements and limitations for user interaction. It should also specify possible failure conditions and explain how to handle each case.


A specification template must include clear milestones pre-defined and agreed on by all parties involved. These milestones provide clear metrics toward completion and, in some cases, might also include billing thresholds. You can define milestones in terms of functionality, components, or separate applications in a suite of deliverables.

Learn more in our detailed guide to software documentation requirements (coming soon)

How to write a great software design document

The ideal design document is actively read and updated. Here are several ways you can use to make the SDD reading experience engaging for users:

Make it collaborative and invite feedback

Invite all parties involved with the project to contribute from the beginning. Establish a collaborative process that fosters discussions early on to save informing everyone later. You can leverage collaborative documentation tools to facilitate engaging workflows and enable team members to comment on documents to point out omissions and errors.

Don’t write it in Word

The software development lifecycle is an agile and collaborative process, typically involving feedback loops and reviews to foster a successful outcome. Word has great advantages for certain use cases, but its rigid and closed nature does not facilitate collaborative work.

Effective software design documentation requires open, organized, and collaborative tools. Ensure your SDD is easily accessible to all parties involved by keeping it in a shared drive or an internal knowledge base that is easily searchable and discoverable.

Don’t make it too long or complicated

There is no standard length for SDD. However, the longer the document, the more effort it requires to update and read. The ideal design document is under five pages and uses clear, simple language to describe the solution.

Leverage software design documents as internal documentation

As SDDs include detailed explanations of why a certain code area has been created, as well as how it is constructed and the technical motivations behind its architecture, these documents are invaluable not only during the initial coding process but also later. In other words, these documents are great resources for engineers who want to contribute to the relevant code parts, regardless of whether they have been part of the initial implementation efforts.

When using Swimm, teams can couple the SDD with their code, thereby making sure the SDD remains up-to-date as the code evolves, making sure the information within the SDD is always relevant.

Think about future readers and provide sufficient context

In order to be really valuable, your document needs to be read and understood by someone else. This someone is different from you, meaning they don’t have all of the context you hold in your head – for example, the business logic or constraints taken into consideration when writing this design doc. It’s a good practice to have a section of “Additional Context” or “Assumptions”, dedicated especially for that.

Include software design documents in dev iterations (preferably using docs as code)

Save your document as a markdown file on the feature branch where the feature is about to be implemented. This way it’s accessible to the relevant collaborators, and also for the code reviewers, who’d appreciate the details in the document when performing the code review.

Include information about the actual implementation as it happens

No matter how good your team is at designing, the reality is always stronger than the beautiful plans, and thus the resulting code will probably differ (slightly or not) from the original design. As you implement the code, you should update the document so when the PR is merged, including the code changes as well as the document, the doc truly depicts the state of the code.

Related content: read our guide to technical documentation examples.

Keep documentation up to date

In order for the doc to be relevant in the future, you should keep it in sync with the code as it evolves in future changes.

How Swimm can make this happen

With Swimm, you can make your Design Documents always synced with your code, during and after development of the relevant feature. Simply follow these steps:

  1. Create your Design Document with Swimm. As the code hasn’t been implemented yet, the doc will probably not include references to it (unless you would like to reference existing code parts).
  2. Start a branch (and a corresponding Pull Request) where this Doc is committed.
  3. Implement your code. As you do, add relevant parts of the code to the document and make changes to the corresponding design as you do.
  4. Merge your Pull Request as usual.
  5. Swimm will make sure the document remains up-to-date as the code keeps evolving.

Want to try out Swimm for design documents? Click here to sign up for a 1:1 demo.