The document that kicks off most software projects is also the document that causes most software projects to fail. Technical specifications sound bureaucratic—paperwork that delays the real work of building. In practice, the spec is where projects are won or lost before a line of code is written.

The Purpose of a Spec

A specification isn't a legal document or a formality. It's a shared understanding made concrete. When the spec is good, everyone involved—developers, designers, clients, stakeholders—knows what's being built and can recognise it when it's done.

Without that shared understanding, projects drift. The client imagines one thing, the developer builds another. Neither is wrong—they just had different pictures in their heads. A spec makes the picture explicit so mismatches surface before they become expensive.

What Belongs in a Spec

A good spec answers three questions: what does the system do, who uses it, and how do they use it?

The first question is about functionality. The system should allow users to create accounts, manage profiles, send messages—whatever the core capabilities are. This section should be specific enough to implement but not so detailed that it constrains solutions. "Users can reset their password via email" is good. "A PHP script sends a password reset link using the mail() function" is too specific.

The second question is about users. Who interacts with this system? An admin has different needs than an end user. A mobile user has different constraints than someone at a desktop. Understanding the audience shapes countless implementation decisions.

The third question is about workflows. What sequence of actions accomplishes each goal? A user wants to purchase a product—they browse, select, add to cart, check out, pay. Each step implies interface elements, data requirements, and integration points. Workflows reveal complexity that feature lists obscure.

What Doesn't Belong

A spec should avoid prescribing implementation details unless those details genuinely matter to the outcome. If you don't care whether the backend uses Python or Node, don't specify it. If you don't care whether the database is PostgreSQL or MySQL, leave it open.

Prescribing unnecessary details restricts options without adding value. The developer might know a better approach, or circumstances might change, or the preferred technology might have drawbacks that weren't apparent initially. Leaving room for expertise is different from being vague—it's being appropriately specific.

Similarly, a spec should avoid time estimates. Specifications describe what, not when. Estimates come after the spec is complete, informed by its contents. Embedding timeline assumptions into requirements creates pressure to skip or compromise when reality doesn't match expectations.

The Right Level of Detail

Finding the right level of detail is the core skill of specification writing. Too vague and the spec doesn't constrain enough—developers fill gaps with assumptions that might not match intentions. Too detailed and the spec becomes brittle—any deviation requires renegotiation.

The test is whether someone could implement the spec differently than you imagined while still meeting its requirements. If they could, and that alternative would be acceptable, the spec is probably at the right level. If the alternative would be wrong, the spec needs more detail in that area.

Consider a requirement like "users can search for products." This might be enough if any reasonable search implementation is acceptable. But if search must be instant, must handle typos, must weight certain fields higher than others—those requirements need explicit statement.

Living Documents

Specs should change when understanding changes. Treating a spec as frozen once written creates problems when reality intrudes. Users need something different than anticipated. Technical constraints emerge that weren't visible initially. Market conditions shift.

The discipline is distinguishing between changes that improve the project and changes that inflate its scope. A clarification that better captures original intent is different from a new feature that extends the original vision. The spec should accommodate the former while providing structure to evaluate the latter.

Good change management means documenting what changed and why, so decisions made based on the original spec can be revisited. It means ensuring all stakeholders understand the updated requirements. It means adjusting expectations—timeline, budget, complexity—to match the new scope.

Specs as Communication

The ultimate purpose of a specification is communication. It fails if people walk away with different understandings, regardless of how thorough or well-written it appears. Success means alignment—everyone working toward the same outcome because they share a clear picture of what that outcome looks like.

This means specs should be readable by non-technical stakeholders. Jargon and technical detail have their place, but the core intent should be clear to anyone with a stake in the project. If a business owner can't understand what's being built from the spec, the spec has failed as a communication tool.

Starting a project and want help getting requirements right? Get in touch and we'll work through your spec together to ensure we're building what you actually need.

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