A technical specification is the document that describes how an application will be built: its scope, functional and non-functional requirements, architecture, technology stack, interfaces, milestones and quality criteria. It turns an idea into something a team can estimate, build and test against, and it is the single most effective way to avoid the missed deadlines and budget overruns that come from "we assumed you meant".
This guide explains what a technical specification for an app should contain, how it differs from a software requirements specification and a functional specification, how to write each section, and what changes when the application includes AI features. A section-by-section template and a worked example are included so you can start from a structure rather than a blank page.
TL;DR / Key Takeaways
- A technical specification answers three questions for every feature: what the system does, how well it must do it, and how it will be built and verified.
- Functional requirements belong in user stories with acceptance criteria; non-functional requirements need numbers (response time, uptime, data volumes), not adjectives.
- APIs, data flow, the technology stack and the deployment model are part of the spec, not decisions left to the first sprint.
- AI features need their own section: data sources, evaluation method, guardrails and the legal role your company takes under the EU AI Act.
- AI tools can draft a spec quickly, but every requirement still has to be validated by the people who own the product and the people who will build it.
What is a technical specification and why does it matter?
A technical specification describes how an application should be built. It defines the scope, the requirements, the architecture and the technologies involved, and it records the decisions that shape the product. Without a clear spec, development drifts: deadlines slip, budgets overrun and the delivered product does not match what the business expected.
Think of it as the blueprint for a building. You would not start construction without plans, and you should not start development without a document that everyone has read and agreed. A good specification prevents misunderstandings between stakeholders and engineers, keeps the team aligned when priorities change, and gives the project a reference point for every "is this in scope?" conversation.
It also protects the budget. Most expensive changes in a software project are not caused by hard engineering problems; they are caused by requirements discovered late. A specification moves that discovery to the cheapest moment, before code exists. At WWG it is the first deliverable of every custom software development engagement, and often the output of a short proof of concept when the technical risk is high.
How does a technical specification differ from an SRS or a functional specification?
The three documents overlap but answer different questions. A software requirements specification (SRS) states what the system must do from the user's point of view. A functional specification details how each function behaves. A technical specification adds how the system will be built: architecture, stack, interfaces, environments and testing. Smaller projects merge them into one document.
| Document | Main question | Typical owner | Typical audience |
|---|---|---|---|
| Software requirements specification (SRS) | What must the system do, and for whom? | Product owner or business analyst | Stakeholders, designers, engineers |
| Functional specification | How does each function behave in detail? | Product owner with the tech lead | Engineers, QA |
| Technical specification | How will the system be built, deployed and verified? | Tech lead or architect | Engineering team, DevOps, vendor |
The distinction matters when you buy development from a partner. An SRS is enough to ask for a rough estimate. A technical specification is what a vendor needs to commit to a scope, a timeline and a price, and it is what you will hold them to during acceptance. If the document you have only says what the app should do, expect the estimate to carry a wide range.
What should a technical specification for an app contain?
A complete technical specification for an application has ten sections: purpose and goals, users and roles, functional requirements, non-functional requirements, architecture and technology stack, APIs and data flow, milestones, testing plan, deployment and maintenance, and open questions or assumptions. The table below is the template we use.
| Section | What to write | Common mistake |
|---|---|---|
| 1. Purpose and goals | The problem, the target users, the measurable outcome | Describing features before the goal |
| 2. Users and roles | Each role, its permissions and its main journeys | Forgetting admin, guest and support roles |
| 3. Functional requirements | User stories with acceptance criteria, grouped by module | "The app should be user-friendly" |
| 4. Non-functional requirements | Performance, security, scalability, compatibility, compliance, with numbers | No numbers |
| 5. Architecture and stack | Components, languages, frameworks, databases, third-party services, and why | Choosing by fashion, not by constraints |
| 6. APIs and data flow | Endpoints, request and response formats, authentication, error handling, data model | Leaving the API to "be defined in the sprint" |
| 7. Milestones | Phases, deliverables, dependencies and a contingency | A timeline with no buffer |
| 8. Testing plan | Test levels, tools, environments, success criteria | Testing mentioned once, in the last line |
| 9. Deployment and maintenance | CI/CD, hosting, monitoring, release cadence, rollback, ownership after launch | No plan for who fixes bugs after go-live |
| 10. Assumptions and open questions | Everything not yet decided, with an owner and a date | Hiding uncertainty in confident prose |
Length is not the goal. A specification for a three-month project can be fifteen pages; the important thing is that every section is answered, even when the answer is "not applicable, because...".
How do you define functional requirements that developers can build?
Functional requirements describe what the application does, written so that a developer can build them and a tester can verify them. The reliable format is a user story with acceptance criteria: who wants what, why, and how you will know it works. Vague statements such as "the app should be intuitive" are not requirements; they are wishes.
Start from the purpose. Suppose you are building a food delivery app. The main goals might be to let customers order quickly, to give them real-time tracking, and to let restaurants manage orders efficiently. Each goal breaks down into concrete features:
- Customers can create an account with an email address or a social login.
- The app supports secure payment methods, including cards and digital wallets.
- Restaurants can update their menu and availability in real time.
Then write each feature as a story with criteria. "As a customer, I want to save multiple delivery addresses so that I can order to home or office" becomes testable when you add: an address has a label, street, city and postcode; a customer can store up to ten; the last-used address is preselected at checkout; deleting an address does not affect past orders.
Describe workflows, screen interactions and expected behaviour for each user role (administrator, registered customer, guest, restaurant staff) and their permissions. Include the edge cases in the same place: what happens when a payment fails, when a restaurant goes offline mid-order, or when a user enters an invalid postcode. Edge cases written now cost minutes; discovered in production they cost a release.
Which non-functional requirements must be specified?
Non-functional requirements define how well the system performs rather than what it does: performance, security, scalability, availability, compatibility and compliance. Each one needs a measurable target, because "fast" and "secure" cannot be tested. They are the requirements most often skipped, and the ones most often behind a failed launch.
Write them as targets with a number and a condition:
- Performance: the home screen loads within two seconds on a 4G connection; search results return within 500 milliseconds for a catalogue of 50,000 items.
- Security: all personal data is encrypted in transit and at rest; authentication uses short-lived tokens; the application meets the relevant level of the OWASP Application Security Verification Standard, which the OWASP project publishes as a basis for testing web application security controls and as a list of requirements for secure development. (owasp.org)
- Scalability: the system supports 100,000 monthly active users and 2,000 concurrent sessions without degradation, and the architecture can scale horizontally.
- Availability: 99.9% monthly uptime for the ordering flow, with a recovery objective of one hour.
- Compatibility: iOS 16 and later, Android 12 and later, the last two versions of the major desktop browsers.
- Compliance: the GDPR requires data protection by design and by default (Article 25), so data minimisation, retention periods and the lawful basis for each data category belong in the spec, not in a policy written later. (eur-lex.europa.eu)
Add disaster recovery: what is backed up, how often, where, and how long a restore takes. If the application handles payments or health data, name the additional standards that apply and who is responsible for the audit.
How do you specify the stack, APIs, milestones and testing?
The technical half of the document turns requirements into an engineering plan. Specify the technology stack with reasons, describe every API and the data flow between components, break the work into milestones with deliverables, and define how each level of testing will prove the requirements. These four sections are what let a team estimate with confidence.
Technology stack. List languages, frameworks, databases, hosting and third-party services, and say why each was chosen. For example: React Native for a cross-platform mobile client; Node.js with a typed framework for the backend; PostgreSQL for transactional data and Redis for caching; a cloud provider with auto-scaling for hosting. Community support, hiring availability, scalability and maintenance cost are the criteria that matter; novelty is not.
APIs and data flow. Define endpoints, request and response formats, authentication (JWT, OAuth 2.0), error codes, rate limits, validation rules and retry behaviour. Describe how data moves between the client, the backend, the database and external services such as payment or maps providers. The OpenAPI Specification is the standard, language-agnostic format for describing HTTP APIs; writing the API contract in it means the frontend and backend teams can work in parallel against the same definition. (spec.openapis.org)
Milestones. Break the project into phases with clear deliverables, for example: weeks 1–2 requirements sign-off and UX design; weeks 3–6 core backend services and database; weeks 7–10 frontend and API integration; weeks 11–12 testing, fixes and deployment. Add dependencies (design before frontend, payment provider account before checkout) and a contingency for the unknowns you listed in section 10.
Testing and quality assurance. State which test levels apply and what success looks like: unit tests for individual functions, integration tests for components working together, end-to-end tests for the main journeys, performance tests for the load targets, and user acceptance testing with real users. Name the tools and environments. A chat feature, for instance, is not done until it has been tested with 10,000 simultaneous messages and the latency target held. This is the section a QA team will use to plan its work, so write it with them.
Deployment and maintenance. Describe the CI/CD pipeline, the hosting model, monitoring and alerting (crash reporting, performance, uptime), the release cadence, the rollback procedure and who owns bug fixes after launch. An application without a maintenance owner starts accumulating technical debt on day one.
What does a worked example look like?
A worked example shows the level of detail a good specification reaches. The excerpt below is for one feature of the food delivery app, live order tracking, written in the format described above. Every requirement has a number, a condition and a way to verify it; the API contract and the test that proves it sit next to the requirement.
| Element | Specification |
|---|---|
| User story | As a customer, I want to see where my order is so that I know when to expect it. |
| Acceptance criteria | Status changes (accepted, preparing, picked up, delivered) appear within 5 seconds; the courier position updates every 10 seconds while "picked up"; the screen works without the map if location permission is denied. |
| Non-functional | Tracking data is retained for 30 days, then deleted; position updates use TLS; the tracking endpoint supports 2,000 concurrent connections. |
| API | GET /orders/{id}/tracking returns status, timestamps and an optional courier position; WS /orders/{id}/live streams updates; 401 without a valid token; 404 for orders the customer does not own. |
| Dependencies | Courier app sends positions; maps provider account; push notification service. |
| Tests | Integration test for each status transition; load test at 2,000 connections; manual test of the denied-permission path on iOS and Android. |
| Open questions | Should restaurants see the courier position? Owner: product. Decide by end of week 2. |
Multiply this by the number of features in scope and you have the body of the document. Most of the value is in the "open questions" row: it is where the conversations that would otherwise happen in month three happen in week one.
How do you handle AI features and AI-drafted specifications?
AI changes the specification in two ways. If the application includes AI features, the spec needs a section on data, evaluation, guardrails and legal role, because an AI feature without acceptance criteria cannot be signed off. If you use AI tools to draft the specification itself, the draft is a fast first version that product and engineering still validate.
Specifying an AI feature. For each AI-powered function (a recommendation, a classifier, an assistant), write down: the data it uses and where it comes from; the expected quality, expressed as a measurable evaluation on a test set rather than "accurate"; the fallback when the model is unavailable or uncertain; the guardrails on inputs and outputs; the logging needed to explain a decision later; and the cost ceiling per request. Then classify the feature under the EU AI Act, Regulation (EU) 2024/1689, whose obligations depend on whether your company is a provider or a deployer of the system and on the risk category of the use case. (eur-lex.europa.eu) Our guide to EU AI Act deployer obligations covers the deadlines that apply from 2026.
Using AI to draft the spec. Language models are good at turning a structured brief into a first draft of the ten sections, at proposing edge cases you have not listed, and at converting requirements into user stories with acceptance criteria. They are poor at knowing your constraints: the legacy system you must integrate with, the compliance regime you operate under, the budget and the team you actually have. The working pattern is to generate, then review every requirement with the product owner and the tech lead, and to run the same review for anything the model added on its own. A specification is a set of commitments; a model cannot make them for you.
What are the most common mistakes in a technical specification?
The most common mistakes are vague requirements, missing non-functional requirements, no version control on the document, engineers consulted too late, over-long documents that nobody reads, and edge cases and error handling left out. Each one is cheap to fix while writing and expensive to fix once the code exists.
- Vague requirements. "User-friendly" and "fast" are not criteria. Define the exact expectation and how it will be measured.
- Ignoring non-functional requirements. Teams that specify only features discover performance, security and scalability in production.
- No version control. A spec is a living document. Keep it in a repository or a versioned wiki, with a change log, so everyone knows which version was estimated and which was built.
- Engineers consulted too late. Involve the people who will build the system while the document is being written; they will catch infeasible requirements and hidden integration work.
- Overcomplication. Detail is good; jargon and repetition are not. If a section does not help someone build or test, cut it.
- Skipping edge cases and error handling. Always describe what happens with invalid input, a failed payment, a lost connection or a server outage.
- No owner for open questions. Every unknown needs a name and a date, or it becomes a surprise.
If an existing application has no specification at all, the fastest route back to control is usually a software audit: it reconstructs the actual requirements and architecture from the code, and gives you the document you should have started with.
Frequently Asked Questions
What is the difference between a technical specification and a functional specification?
A functional specification describes how each function of the application behaves from the user's point of view. A technical specification includes that behaviour but adds how the system will be built: architecture, technology stack, APIs, environments, milestones and testing. Small projects often combine both in one document; larger ones keep them separate with cross-references.
How long should a technical specification for an app be?
Long enough to answer all ten sections, and no longer. A three-month mobile app typically needs ten to twenty pages; an enterprise platform with many integrations can need fifty or more. Judge completeness by whether an engineer can estimate from it and a tester can verify against it, not by page count.
Who should write the technical specification?
The product owner writes the goals, users and functional requirements, and the tech lead or architect writes the architecture, stack, APIs, testing and deployment sections. Both review the whole document. When development is outsourced, the vendor's tech lead usually drafts the technical sections during discovery and the client signs them off before the build starts.
Can a technical specification change during development?
Yes, and it should when new information appears, but changes must be versioned and agreed, not improvised. Record what changed, why, and the impact on scope, timeline and cost, then re-baseline the estimate. A specification that never changes is usually one nobody reads; one that changes silently is one nobody trusts.
Do you need a technical specification for an MVP?
Yes, a shorter one. An MVP still needs goals, users, the priority journeys as user stories, the non-functional targets that matter for a pilot, the stack and a test plan. It can skip secondary features. Our MVP guide covers cost and timeline.
Contact us if you want a technical specification written or reviewed before you commit a budget to development.
Sources
- OWASP Foundation, Application Security Verification Standard project page — https://owasp.org/www-project-application-security-verification-standard (owasp.org)
- OpenAPI Initiative, OpenAPI Specification (latest version) — https://spec.openapis.org/oas/latest.html (spec.openapis.org)
- EUR-Lex, Regulation (EU) 2016/679 (General Data Protection Regulation), Article 25 — https://eur-lex.europa.eu/eli/reg/2016/679/oj (eur-lex.europa.eu)
- EUR-Lex, Regulation (EU) 2024/1689 (Artificial Intelligence Act) — https://eur-lex.europa.eu/eli/reg/2024/1689/oj (eur-lex.europa.eu)
