Technical Design Document

In software development, we often move fast—especially in agile environments. But moving fast doesn’t mean skipping structure. One of the most important tools to ensure alignment, clarity, and quality is the Technical Design Document (TDD). 

It’s not just for architects or senior engineers; it’s something every developer should know how to write. A well-written design document saves time, reduces misunderstandings, and ensures the team is building the right solution for the right business problem.

Why a Technical Design Document?

Whenever we need to introduce a new system component into an existing system or change the current architecture, it must be done in a way that aligns with company policies. 

In a waterfall setup, this document is usually written only by an architect with many years of experience. But in an agile environment, it’s often created by any member of the team. That means even someone with just a year of experience should know how to write it.

What Needs to Be Included in the TDD

I believe the TDD should have 3 sections, and each one should be finalized and approved before moving on to the next.

1. Introduction Section

The programmer needs to define the scope of the problem and explain why it’s an issue—mainly from the business perspective, not the technical one. 

If the problem has no business impact, why solve it? Some problems may not seem to have a direct business effect (like poor database design or spaghetti code), but in reality they do—causing timeouts, bugs, and delays in adding new features. This section should determine whether the reader continues with the document or not. 

Highlight the business requirements and translate them into a checklist that any solution must satisfy to be acceptable. I recommend finalizing this section before diving deeper, since changes here can affect the entire document.

Good examples: lack of systemic communication with users, poor retention rate, 1 out of every 3 orders failing permanently due to database errors, or order processing time being 12 minutes slower than the market average.
Bad examples: bad DB schema with many nulls, code being hard to onboard or edit, frequent deployment failures.

2. Solution Discussion

A design document should propose a single solution. There may be many possible approaches, but you need to compare them, pick the best one, and justify your choice. Once you’ve selected a solution, dive deeper and describe possible sub-solutions.

For example, if your solution is to design a microservice, then sub-solutions could cover which database to use, how the service communicates with others, and what the abstract interface looks like. 

Another example is migrating a service—sub-solutions here might include how to handle outage time, and how to sync old data with the new system.

This section should also include the high-level flows. Finally, explain how this solution fully addresses the problem, ensuring that every item in the checklist is covered.

3. Timeline and Components Discussion

After agreeing on the solution, it’s important to write a section that breaks the work into features, assigning each with estimated time, priority, and whether it depends on another phase. 

A common mistake here is dividing the work vertically (layer-based) instead of horizontally (feature-based). Remember, in agile we want to deliver as soon as possible. If you build the entire database layer, then the business layer, but don’t deliver a single usable UI or API, you’ll only get feedback very late in the process—and that will backfire.

This section also gives stakeholders the space to decide whether this solution should be developed right now or if other priorities come first—meaning it might be delayed.

Final Thoughts

At the end of the day, a technical design document is not just paperwork—it’s a communication tool. It aligns engineers, product managers, and stakeholders on why a problem matters, how we plan to solve it, and what it will take to get there. Writing a good one takes practice, but it’s a skill every engineer should develop early. It’s one of the best ways to ensure we’re building solutions that are not only technically sound but also meaningful for the business.

Another big benefit is onboarding: when each part of your system has a document describing what it is, why it exists, and how it works, new team members can ramp up much faster.