Why you need a requirements specification
A requirements specification is an agreement between the client and the development team about what exactly will be built. Without it, everyone understands the task differently: the client expects one thing, developers build another, and acceptance turns into an argument. A good spec fixes functionality, constraints and completion criteria, so it directly affects budget and timeline: the more precise the requirements, the fewer reworks and the closer the estimate is to reality. That is why the question of how to write a specification should be solved before discussing price and deadlines, not after.
Structure of a specification: what it consists of
The structure of a specification is almost always the same; only the depth of detail changes. The baseline set of sections is: goal and business context; user roles and scenarios; functional requirements (what the system does); non-functional requirements (load, security, speed, supported devices); integrations with external services; data description and storage; design and interface requirements; stages and acceptance criteria. You do not have to write every section in equal depth - what matters is that each point can be verified: done or not done.
A bad requirement versus a good one
The main mistake in a spec is vague wording that cannot be checked. Bad: "the app must work fast." Good: "the catalog screen opens in no more than 2 seconds on 3G with a list of 500 items." Bad: "convenient login." Good: "phone-number login with an SMS code, code resend after 60 seconds, lockout after 5 failed attempts." The difference is that the second versions are measurable: they make it unambiguous whether the requirement is met, leaving no room for dispute at acceptance.
Specification template: copy the structure for your project
A ready skeleton you can take as a basis and fill in for your task. 1. Project goal - what problem we solve and for whom. 2. Users and roles - who does what in the system. 3. User scenarios - step by step, from entry to the target action. 4. Functional requirements - a list of features, each measurable. 5. Non-functional requirements - load, speed, security, platforms. 6. Integrations - payments, notifications, maps, analytics, external APIs. 7. Data - what we store, where, retention and privacy requirements. 8. Interface - mockups or design links, tone, branding. 9. Stages and acceptance - how we split the work and by which criteria we accept it. Even a brief pass through this template removes most future questions to the team.
Common mistakes in a specification
The first mistake is writing "how to build it" instead of "what the result should be": choosing technologies is not the client's job - the result and constraints are. The second is mixing mandatory and nice-to-have: mark what is needed for launch and what can wait for version two. The third is forgetting non-functional requirements: without stated load and data requirements the team assumes the minimum, and reworking it later costs more. The fourth is not describing edge cases: what happens on a failed payment, lost connection, empty list. Unhandled scenarios are exactly what surfaces at acceptance most often.
What to do if you have no spec and no idea where to start
If you have an idea but neither a spec nor a clear sense of scope - that is normal, and you should not write a detailed document right away. In that situation the answer is a discovery phase: together with you the team turns the idea into a prototype, a requirements backlog and a grounded estimate. Discovery is essentially the process whose output is the specification - but one tested for realism rather than written blind. If you are still choosing a contractor, notice whether they are ready to start with analysis rather than immediately naming a price for "development" without any document.