How to Write a Technical Specification Document

"Go I Know Not Whither and Fetch I Know Not What" © "The Yellow Fairy Book" (1894) 

Up to 70% of projects fail due to poorly defined requirements. Our experience shows that correcting errors after release is 4-5 times more expensive than addressing them during the task formulation stage. Moreover, a development project with clear requirements is 2.5 times more likely to succeed.

Therefore, there’s no point in even starting an argument about the significance and importance of a Technical Specification Document (TSD). It’s a crucial component of development - literally like a map and compass to “go THERE and do THAT.”

What we should really discuss are issues related to the document’s content, its purpose, and the extent to which you (as the client) need to be involved. It’s essential to prepare for these aspects in advance to facilitate smoother interactions and make the development process more productive.

In this article, you’ll learn:  

• who should fill out the TSD;  
• where, when, and how this information will be used;  
• how to describe a task if you’re unsure about the technical details;  
• the difference between functional and non-functional requirements, and much more.  

You’ll also receive helpful tips to reduce risks, optimize resources, and improve development quality. Sounds like a solid reason to take the time to read this article, doesn’t it?

What Is a Technical Specification Document?

This is a detailed document that outlines the technical aspects of a project: system architecture, functional and non-functional requirements, interfaces, algorithms, and other technical details.

Below, you’ll find a Technical Specification Document example.

This is a hypothetical example that could be used in the development of an online shopping web application. Typically, a document outline also includes general project information, goals, stakeholders, areas of application, constraints, and assumptions.

In other words, a Technical Specification Document is:

• A single source of information and a technical blueprint. It ensures a shared understanding of what needs to be implemented.
• It serves as the foundation for development and testing. Architectural solutions, code, and test scenarios are created based on the TSD.
• It’s a way to manage changes. Allowing for project control and progress tracking.

In this way, a TSD turns abstract ideas and wishes into well-defined requirements, providing a clear roadmap.

Types of Technical Specification Documents

The TSD is not a strictly standardized document. Each example of a technical specification may vary depending on the company, project, or industry.

However, there are several commonly accepted types of specifications:

• Functional Specification Document (FSD)

It describes functional requirements, is business-oriented, and explains what the system should do without technical details.

Focus: Functions, interfaces, and user scenarios.  

Example Use Case: Describing how customers will interact with the application: registration, placing an order, and tracking delivery status.

• System Design Specification (SDS) или Software Design Document (SDD)

It describes the system architecture and is a more technical document than the functional specification.

Focus: Technologies used in the project, databases, modules, and interaction between components.  

Example Use Case: Describing how the system will be built: programming languages, frameworks, database structure, and component interaction via APIs.

• Technical Requirements Document (TRD)

It describes specific technical requirements for the system related to performance, security, scalability, and compatibility.

Focus: Non-functional aspects (e.g., the number of users the system can support), hardware requirements, or network protocols.  

Example Use Case: Specifying technical limitations, such as supporting functionality with a certain data volume or operating on specific platforms.

• Interface Control Document (ICD)

It describes interactions between various systems and modules. It is often used in B2B projects where third-party integration is required.

Focus: Data formats, protocols, methods of interaction via APIs, and descriptions of endpoints.  

Example Use Case: Describing the API for interaction between the frontend and backend or integration with external systems (CRM, ERP, etc.).

Additionally, at various stages of the project, other important specifications may be used, such as:

1. Product Requirements Document (PRD): Outlines the purpose of the product, its target audience, and how to meet their needs.  
2. Test Specification Document (TSD): Describes test scenarios, expected results, and testing methods.  
3. Security Specification Document: Includes requirements for encryption, authentication, authorization, and access control.

For simple projects with clear business requirements, a Functional Specification or PRD may be sufficient. In complex projects, especially in B2B, where integrations and scalability are important, a System Design Specification or Technical Requirements Document may be necessary.

In Agile projects, instead of traditional technical specifications, user stories may be used—short descriptions of system functions from the user’s perspective. This is a less formal approach but helps to adapt requirements more flexibly throughout the development cycle.

Who, How, and When uses TSD 

It may seem that the Technical Specification Document is only needed by the development team to, simply put, understand what they need to do. But that’s not entirely true.

The client (customer) is equally invested in having a quality TSD. At a minimum, it provides a basis for cost/time estimates and confidence that the team has correctly understood all tasks. At most, it serves as a legal safeguard to set clear expectations and protect both parties in case of disputes.

Each group involved in the project uses the TSD in their own way, and the document’s benefits vary depending on their roles and goals:

Investors: While investors are not directly involved in technical aspects, they may use the TSD to assess project progress and ensure funds are being used effectively. Their advantages: transparency and a clear risk assessment.

Product Owner/Technical Product Owner: The TSD serves as a tool for planning and task distribution. It helps monitor completion status, adhere to timelines, budget, and requirements.

The best part? Our latest payment integration project, with a proper TSD, finished two weeks ahead of schedule and under budget. The document forced us to have all those uncomfortable technical discussions upfront, where changes cost us hours instead of weeks. © Technical Product Owner of Wezom

Programmers/QA Testers/UI/UX Designers: For everyone involved in the project’s development, the TSD is a clear action guide. It helps avoid implementation errors and ensures the product aligns with the client’s requirements.

Operations Teams (DevOps, System Administrators): The TSD helps them understand how to set up the infrastructure for system deployment, determine required resources, and maintain operational stability—essentially, how to do their part well.

Less obviously, Technical Specification Documents are often used by business analysts to link business goals with potential technical solutions.

Advantages of TSD for everyone: clarity, transparency, control over the project and its outcomes, and simplification of all development stages.

How to Write a Technical Specification Document 

To start, let’s address the main question: who should create a technical specification document? Do you need to panic, sign up for programming courses, and memorize various technical terms?

Of course not.

Your role is simply to provide the company with information on what you need: a description of the functionality and your expectations for how the system should work. Now is a good time to get familiar with some key concepts.

Filling Out Functional and Non-Functional Requirements

Both functional and non-functional requirements are equally important for the project’s success. Functional requirements define what the system does (provided by you, the client), while non-functional requirements define how it does it. To clarify the difference, let’s compare them in a table:

You do NOT need to describe everything at the code level or create data flow diagrams — that’s the job of the development team or project manager. Typically, you’re expected to answer questions like:

• What problems do you want to solve?
• What are the project goals?
• What’s important for your business?
• Which processes need automation?
• What internal corporate standards need to be considered? and so on.

Practical Steps and Tips 

You can enhance task clarity by illustrating complex concepts with visual elements. They don't need to be artistic masterpieces—a simple flowchart can be invaluable and will earn you praise from the development team as “one of the best clients we've had.”

Here are some additional tips:

• Ask the manager which sections you should fill out: 

Don’t stress too much about the technical details if you’re unsure of your knowledge. Focus on describing the overall picture or request a technical requirements template.

• Provide Business Context: 

Describe in as much detail as possible what’s important for your business, how the system will be used, and what results you expect.

• Collaborate with the Team:  

Project managers and technical specialists can help you refine your requirements and expectations and translate them into “technical language.” They can also help formulate them correctly if, for example, you’re unsure how to describe roles and access levels for different types of users (like administrators, managers, operators, and external clients).

• Use Examples:  

If you have examples of other services or systems that you like, don’t hesitate to mention them. This will help the team better understand your expectations.

You may have encountered terms in this text that are unfamiliar to you. It’s also quite possible that a typical programmer might not understand what "IOPS" means or how to build an architecture to coordinate business/logistics processes with "MOPS". (Intermediate Oil Pumping Station and Main Oil Pumping Station)

Explain the Meaning of Specialized Terminology used in your product. Provide clear definitions for any specialized terminology used in your product. This way, each stakeholder can check the meaning when needed rather than having to guess. 

Features of Creating TSD for B2B Projects

In the context of B2B projects, the process of filling out a Technical Specification Document (TSD) differs slightly from B2C projects, but the main principles remain similar. However, there are several specific points to pay attention to:

If your platform serves multiple companies (for example, corporate clients), it’s essential that the specification describes how the system will handle a multi-tenancy model. For instance, explain how data will be separated between organizations so that one company cannot access another's information.

In B2B projects, detailed reporting (such as financial reports and analytical data) and the ability to audit user actions (who did what and when in the system) are often required. It’s important to describe what reports are needed and what data should be included in the monitoring and analytics system.

If your project involves managing contractual obligations, it's essential to outline the process for handling contracts and terms (for example, a system for automatic contract renewals and penalty calculations for delays).

Payments in B2B are usually more complex than in B2C. For instance, the system may require support for deferred payments, integration with banking systems, or the ability to manage corporate accounts.

Conclusion

Filling out a Technical Specification Document is not just paperwork; it's risk management in the form of a document. Treat it like an insurance policy or an opportunity to create a successful project.

One of the best proofs of the importance of a spec document is a story shared at the Product Management Summit 2023 by one of the attendees:

In 2022, we were working on integrating a new payment system into our e-commerce platform. Three months and $180,000 later, I learned a lesson the hard way. The development team built exactly what I asked for—a payment system that accepted international transactions. But without a detailed technical document, they didn't account for the regional payment methods popular in Asia and the transaction rollback mechanisms required by EU regulations. We had to rebuild about 40% of the system.

Here's what happens when a project manager doesn't spend enough time properly and thoroughly filling out the "paperwork" or uses a generic technical specification template without delving into the details. But don’t worry, we guarantee that the specialists at Wezom will carefully ensure that all key points are taken into account. Our team will offer possible solutions and technologies based on your requirements and goals. We'll help you choose the optimal technical solutions to implement any business objectives.

 F.A.Q.

How long will it take to fill out the TSD?

It depends on the scale of the project. If it is large and contains many functional and non-functional requirements, filling it out may take several days. However, we will assist you at every stage to make the process as quick and efficient as possible.

What should I do if my requirements change during the process?

Changes are a normal part of any project. The tech spec document can always be updated, and we can make revisions as needed. However, it’s important to remember that significant changes may affect the project's timeline and budget.

All functions are important for my business; how do I prioritize them?

We will help you categorize your requirements into critical (essential for implementation), important (needed but not urgent), and desirable (additional enhancements). This will help allocate resources effectively and focus on the core tasks at the initial stage, while improvements can be implemented gradually.