How to write a requirements specification for your app

By Weapp · Updated 2025-05-10

A good requirements specification for an app describes purpose, target audience, key user flows, integrations, and non-functional requirements like performance and security, in 5–15 pages. It should be clear enough for comparable bids but describe what the app needs to achieve, not lock in how the solution gets built. The level of detail grows with the project's phases.

The requirements specification has one job at the bidding stage: getting several vendors to price the same thing, without locking in a solution that turns out to be wrong. Most specs fail in one direction or the other, either so vague that the bids become guesswork, or so over-specified that the vendor’s expertise never gets used. Here’s how to find the right level.

The structure template: six sections that go a long way

A spec for the bidding stage rarely needs to be more than 5–15 pages, structured roughly like this:

Purpose and goals. What problem does the app solve, for whom, and how will you know if it succeeds? A couple of measurable goals (say, the share of cases that shift from phone to app) will steer a thousand detail decisions later.
Target audience and usage context. Who are the users, how often do they use the app, and in what environment? An app used daily by field staff wearing gloves has different requirements than one opened on the couch once a month.
User flows. Describe the 5–10 most important things a user should be able to do, as flows: “the user photographs the receipt, selects a project, the approver is notified.” Prioritize them as must/should/could, it’s the prioritization that makes bids comparable and phasing possible.
Integrations. Which systems should the app talk to, business systems, login, payment, maps? State what already exists (APIs? documentation?) for each system. Integrations are one of the biggest cost drivers, and unknown integrations are the most common source of blown budgets.
Non-functional requirements. Performance, security, GDPR, accessibility, offline support, language, platforms, and versions. Only write requirements you’re serious about, every “the app should work offline” costs real money.
Constraints and assumptions. Budget range, desired timeline, who on your side makes decisions, what happens after launch (maintenance, further development).

The mistakes that make bids incomparable

When three bids on the same spec differ by a factor of three, it’s usually the spec, not the vendors, that’s the problem. The most common causes:

Features without prioritization. If everything seems equally important, one vendor prices all of it, another a reasonable subset, and the totals become impossible to compare.
Invisible integrations. “Connect to our business system” without saying which system, which version, or whether an API exists. One vendor assumes the easy case, another allows for the hard one.
Solution requirements disguised as needs. “The app needs a chat feature,” or is the actual need for users to get fast answers? An FAQ plus notifications might be enough. Describe the need, leave the solution open.
Undefined terms. “Simple administration,” “high security,” “fast.” Attach numbers or examples to everything you can: what should an administrator be able to do, which data is sensitive, how fast is fast?
No mention of what already exists. Existing design, brand guidelines, backend, accounts? What already exists shouldn’t get priced again.

A good test: can two people read the spec and sketch roughly the same app on a whiteboard? If not, clarify the flows.

The right level of detail at the right phase

The requirements spec isn’t a document that’s finished once; the level of detail should grow with the project:

Idea phase: one page. Problem, target audience, business idea. Enough to test the idea against colleagues and get rough price indications.
Bidding phase: 5–15 pages per the template above. Clear flows and constraints, open solution. This is where comparability is decided.
Kickoff phase: the spec gets broken down together with the chosen vendor into a prioritized backlog, sketches, and technical decisions. Now the details can come, with the developers’ and designers’ knowledge in the room.
During development: requirements get refined phase by phase. Detailing flow 8 before flow 1 is built is wasted effort, you’ll learn things that change the plan.

Writing the entire detailed spec up front feels safe but rests on the illusion that you already know everything. The best projects lock in the target picture early and the details late.

Get started

Start with purpose, target audience, and the five most important flows, that’s 80 percent of the value. Want a second pair of eyes on your spec, or help producing it through a short discovery phase? At Weapp, we do this regularly for clients, from MVPs to large systems apps, get in touch and we’ll look at it together.

Frequently asked questions

How long should a requirements specification be?

For the bidding stage, 5–15 pages is usually enough. Shorter than that, and the bids become guesswork; longer than that, and you've probably started designing the solution for the vendor. The details then get filled in during the project, flow by flow, together with the team building it.

Do I need sketches or design in the spec?

Simple sketches of the key screens help more than long text descriptions, even hand-drawn ones. They show what you mean and expose gaps in the thinking. But don't commission finished design before the vendor is chosen; design is part of the work you're buying and is best shaped together.

What are non-functional requirements?

Requirements for how the app should work rather than what it should do: performance, accessibility, security, GDPR compliance, offline behavior, scalability, and which platform versions need support. They're often forgotten in specs but drive a large share of the cost, so vendors who don't get them make different assumptions.

Should I require a specific technology in the spec?

Only if you have a real reason, such as an existing codebase or in-house skills that will handle maintenance. Otherwise, describe the need and let vendors justify their technology choices in the bid. How they reason about the choice says a lot about their seniority, and you avoid locking in the wrong solution.

Can the vendor help write the requirements specification?

Yes, many agencies offer discovery phases where the requirements get worked out together. That often produces a better spec than writing it alone, but do it with a party you'd be willing not to hire for the build, otherwise the spec easily ends up shaped around their particular solution. Ownership of the document should be yours.