The verdict in three sentences
A B2B SaaS requirements spec is not a technical document: it is the contract that freezes scope, separates the core from options and avoids the 40% budget drift typical of poorly scoped projects. Its backbone is eight modules — multi-tenant, authentication & roles, core domain, billing, API, admin, notifications, observability — each priced and tied to a milestone. Done well, it turns a vision into a signable quote and a delivery plan in 3 to 5 batches.
The 8 modules of a SaaS spec and their budget
2026 order of magnitude for a B2B SaaS built by an agency, excluding advanced design and third-party licences.
| Module | Key content | Effort (person-days) | Budget EUR excl. VAT |
|---|---|---|---|
| Multi-tenant | Data isolation, org onboarding | 15-25 | 10,000 – 18,000 |
| Auth & roles | SSO, RBAC, invitations, MFA | 12-20 | 8,000 – 15,000 |
| Core domain | Domain objects and workflows | 30-60 | 22,000 – 48,000 |
| Billing | Subscriptions, quotas, Stripe/Wave | 15-30 | 11,000 – 24,000 |
| Public API | REST/Webhooks, keys, docs | 12-25 | 9,000 – 20,000 |
| Back-office admin | Client/plan management, impersonation | 10-18 | 7,000 – 14,000 |
| Notifications | Email, in-app, digest | 6-12 | 4,000 – 9,000 |
| Observability | Logs, metrics, alerts, GDPR | 8-15 | 6,000 – 12,000 |
A complete spec sums these lines and shows a realistic core total of EUR 77,000 to 160,000 excl. VAT before options. Each module must state nominal cases, error cases and measurable acceptance criteria.
Structuring the document and milestones
A good spec follows a stable template that team and client read the same way.
| Spec section | What it freezes | Control deliverable |
|---|---|---|
| Context & goals | Problem, success KPIs | 1 approved page |
| Personas & roles | Who does what, permissions | RBAC matrix |
| User stories | Prioritised functionality (MoSCoW) | Estimated backlog |
| Data model | Entities, relations, tenant | Relational schema |
| Integrations | ERP, payment, email, SSO | List + API contracts |
| Non-functional | Perf, security, GDPR, SLA | Quantified criteria |
| Milestones & QA | Batches, dates, acceptance | Delivery plan |
Classic milestones: Batch 1 multi-tenant core + auth (week 4), Batch 2 core domain (week 10), Batch 3 billing + API (week 14), Batch 4 admin + observability + QA (week 18). Each batch is paid on acceptance, protecting both parties.
Mini case study
Sophie, COO of a B2B HR software vendor in Lyon, arrives with a 2-page brief. After discovery, the spec prices the core at EUR 124,000 excl. VAT (145 person-days at EUR 720 + integrations). She applies MoSCoW: the public API and digest notifications move to "Could" (phase 2), removing EUR 29,000. The signed scope drops to EUR 95,000 excl. VAT across 4 batches. Measured result: zero change orders over the first 14 weeks, versus an average of 2 to 3 change orders (EUR 18,000) on her previous projects without a formal spec.
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FAQ
Who should write the requirements spec, the client or the agency?
Ideally both: the client brings the domain and KPIs, the agency structures, prices and challenges. A paid discovery phase of EUR 5,000 to 12,000 excl. VAT prevents tens of thousands in later drift.
How many pages should a B2B SaaS spec be?
Budget 20 to 45 useful pages: beyond that it becomes unreadable; under 15 it leaves too much implicit. Quality comes from measurable acceptance criteria, not volume.
How do I avoid costly change orders?
By prioritising with MoSCoW and locking the "Must have" core before any development. Each scope change is then priced separately, typically EUR 800 to 3,000 excl. VAT per enhancement.
Is multi-tenant essential from the start?
For a B2B SaaS serving multiple clients, yes: retrofitting data isolation costs 2 to 3 times more than an upfront design, i.e. EUR 20,000 to 45,000 excl. VAT of avoidable cost.
How long to produce a solid spec?
Two to four weeks of workshops and writing for a mid-size SaaS, costing EUR 5,000 to 12,000 excl. VAT — the highest-ROI investment of the project.
Let's scope your project. Send us your brief and target KPIs: we produce a spec priced by module, with an indicative budget and a batched delivery plan. Detailed quote within 48 h. WhatsApp +221 77 596 93 33.
Mohamed Bah
Fondateur, Kolonell
Passionate about digital and entrepreneurship in Africa, Mohamed has been helping Sénégalese businesses with their digital transformation since 2020. Founder of Kolonell, he believes every SME deserves a professional and accessible online présence.
