Design Document
A design document is a written plan in which a development team records, before coding begins, what is to be built and why exactly in that way. It describes the goal, the approach, rejected alternatives, and open risks, and is commented on by colleagues before work starts.
Before a team builds a larger piece of software, someone writes down exactly what is supposed to be created. This document is called a Design Document, roughly translated as a draft or design document. It contains the problem, the planned approach, and the reasoning behind it. Equally important is what one deliberately chooses not to do and which other paths were examined and discarded. The document is usually five to twenty pages long and is not a contract, but a basis for discussion. Others on the team read it, add comments, and ask uncomfortable questions while nothing has been programmed yet.
Why teams write first and build afterward
A mistake in the plan costs almost nothing. The same mistake in finished code costs weeks. Anyone who realizes only after three months of work that the chosen database cannot handle the expected load has to rebuild large parts from scratch. A design document brings such questions forward, into a phase where replanning simply means rewriting a paragraph.
The second benefit is the act of writing itself. An idea can seem coherent in one’s head and fall apart once put into words. As soon as one has to formulate a process step by step, gaps become apparent. Experienced developers therefore say the document is above all a tool for thinking, and only after that a tool for communicating.
Then there is the team’s memory. In two years, someone will ask why a component was built in such a strange way. Without a document, all that remains is guesswork or an expensive rebuild. With a document, one finds the original reasons, including the conditions that may no longer apply by then. This is precisely the point that makes the difference between a well-justified rebuild and blind trial and error.
What such a document contains
Almost all design documents follow a similar structure. At the beginning comes the context: What is the problem, who has it, why now? This is followed by the goals and, explicitly, the non-goals as well. A non-goal is something one deliberately leaves out, so that later no one silently expects it.
The main part describes the design. What building blocks exist, how do they talk to each other, how does the data flow? Rough figures are often included here: expected requests per second, storage requirements, cost per month. Such estimates don’t need to be exact. They are meant to show whether an idea is off by a factor of ten, and that only becomes apparent once you do the math.
A separate section is devoted to alternatives. It states which other solutions were examined and why they failed. At the end come risks, open questions, and a rough timeline. It is important to note the difference from technical documentation: a design document looks forward and describes an intention. Documentation looks backward and describes how the finished system actually works.
From everyday life at Google to AI labs
Large software companies like Google, Amazon, or Microsoft have worked this way for decades. At Amazon, a strict variant applies: instead of slides, one writes continuous prose, which is read silently at the start of a meeting. Only afterward is it discussed. In the open-source world, similar papers are often called RFCs, short for Request for Comments. New features in programming languages like Python or Rust also go through such publicly readable drafts.
In AI companies, design documents are now just as common as in traditional software development. A training run for a large language model can cost millions and take weeks. A poorly thought-out setup cannot simply be corrected on the fly. That’s why, beforehand, it is recorded which data will be used, how large the model will be, and by what criteria success will be measured.
A common misconception is that design documents are bureaucracy that slows down work. In practice, a rule of thumb based on effort applies instead: for a two-day change, a document isn’t worthwhile. For a project spanning several months with many people involved, a few days of writing are almost always cheaper than the first major wrong turn. Anyone who reads in job postings that good written communication is required will find one of the main reasons for that here.
