CUSTOM SOFTWARE REQUIREMENTS
Writing a Software Requirements Document That Works
Bottom Line Up Front (BLUF)
A software requirements document (SRD) is the single artifact that determines whether your custom software project succeeds or fails. 80% of project failures trace back to incomplete or ambiguous requirements, not bad code. The SRD must specify user roles and permissions, core workflows step by step, integration points with existing systems, the data model with entity relationships, and non-functional requirements including performance, security, and compliance. If your vendor starts coding without a completed SRD, you are funding their guesswork.
You would not build a commercial building without architectural drawings. Yet businesses routinely commission $50K-$200K custom software projects with nothing more than a verbal description and a few bullet points in an email. The software requirements document is your architectural drawing. Without it, your developer is making assumptions about your business logic that will cost you $150-$300 per hour to correct after the code is written.
The 5-Section SRD Template
Section 1: User Roles and Permissions
List every person who will use the system and define exactly what they can see and do. Be exhaustive. Examples by industry: Property Management: property manager (view all properties, create and edit leases, approve maintenance, generate reports), tenant (view their lease, submit maintenance, view payment history, make payments), investor (view portfolio performance, download reports, no edit access). Medical Practice: physician (view patient records, sign orders, review lab results), nurse (update vitals, manage patient queue), receptionist (schedule appointments, verify insurance, process intake), billing clerk (submit claims, post payments, manage denials). Construction: project manager (view all projects, approve change orders, run reports), superintendent (submit daily reports, manage RFIs, view schedules for their site only), estimator (create and edit bid packages, view material pricing).
Section 2: Core Workflows (Step by Step)
For each major workflow, document the exact sequence of events including trigger, steps, decision points, and completion criteria. Example: Lease Renewal Workflow. Trigger: system detects lease within 90 days of expiry. Step 1: system generates renewal offer using current market rate template. Step 2: system emails offer to tenant with 15-day response deadline. Step 3: tenant accepts, counters, or declines via portal. Step 4A (accept): system updates lease record, generates new payment schedule, notifies property manager. Step 4B (counter): system notifies property manager for manual negotiation. Step 4C (decline or no response after 15 days): system escalates to property manager with vacancy risk alert. Every workflow must be documented at this level of specificity.
Section 3: Integration Map
For each external system the software must connect to, document five attributes: system name, data direction (read, write, or both), specific data fields exchanged, API availability and version, and authentication method. Example: QuickBooks Online: direction is write, push monthly rent receipts as journal entries, fields are date, amount, account code, tenant name, API is REST v3, authentication is OAuth 2.0. Example: Epic EMR: direction is both, read patient demographics and schedules, write visit notes and orders, API is HL7 FHIR R4, authentication is SMART on FHIR. If a critical system has no API, document that explicitly. The developer needs to know upfront that integration will require web scraping, file-based exchange, or manual workarounds.
Section 4: Data Model
Define your core entities and their relationships. For property management: Properties contain Units, Units contain Leases, Leases link to Tenants, Tenants generate Payments, Properties have Maintenance Requests linked to Units and Vendors. For each entity, list the required fields and their data types: Lease entity requires tenant_id (foreign key), unit_id (foreign key), start_date (date), end_date (date), monthly_rate (decimal), security_deposit (decimal), status (enum: active, expired, pending_renewal). This prevents the developer from making assumptions about your data structure that you will pay to correct later.
Section 5: Non-Functional Requirements
Performance: page load under 2 seconds, report generation under 5 seconds. Availability: 99.9% uptime (8.7 hours maximum downtime per year). Security: AES-256 encryption at rest, TLS 1.3 in transit, role-based access control, multi-factor authentication for admin accounts. Backup: daily automated backups with 30-day retention, cross-region replication for disaster recovery. Compliance: specify HIPAA, SOX, PHMSA, state-specific tenant data privacy laws, or bar association ethics requirements as applicable. Each non-functional requirement must be testable. The vendor should be able to demonstrate compliance against every item in this section.
The Quality Test
Common SRD Mistakes
| Mistake | Consequence | Fix |
|---|---|---|
| Describing features instead of workflows | Developer builds features that do not connect into a usable flow | Document step-by-step sequences, not feature lists |
| Skipping the data model | Developer invents a data structure that does not match your business logic | Define entities, fields, and relationships explicitly |
| Vague integration requirements | Developer discovers API limitations mid-project, causing timeline and budget overruns | Verify API availability and test access before development starts |
| No non-functional requirements | Software works but is too slow, insecure, or unreliable for production use | Specify performance, security, and compliance criteria with testable thresholds |
For a broader pre-build planning framework, see our Pre-Build Checklist. For vendor evaluation criteria, use our Vendor Evaluation Scorecard.
Write it down before you write code.
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