The verdict in three sentences
A functional requirements spec is the only document that turns a fuzzy idea into a project you can price and compare across vendors. In London in 2026, a structured spec cuts budget overruns by 25 to 35 % and halves acceptance disputes. Expect 30,000 to 80,000 EUR HT for a first version of a business application, and never open an RFP without prioritised user stories and written acceptance criteria.
What a web app spec must contain
A spec that protects the buyer clearly separates the what (business need) from the how (technical choices left to the vendor). It describes the context, personas, target journeys, business rules, security constraints and acceptance criteria. It avoids the 90-page unreadable spec trap: 20 to 35 well-prioritised pages beat a brick nobody rereads.
| Spec section | Expected content | Common mistake |
|---|---|---|
| Context & goals | Business problem, target KPIs | Unmeasurable goals |
| Personas & roles | 3-5 profiles, related rights | A single generic profile |
| User stories | "As a... I want..." format | Features listed without value |
| Business rules | Calculations, workflows, statuses | Implicit verbal rules |
| Technical constraints | GDPR, EU hosting, SSO | Stack imposed for no reason |
| Acceptance criteria | Acceptance tests per story | "By eye" acceptance |
| Schedule & milestones | Lots, key dates, penalties | A single delivery date |
Prioritise with MoSCoW and estimate
The MoSCoW method sorts each story into Must, Should, Could, Won't. It makes the budget negotiable: if the envelope is 50,000 EUR HT and the Musts eat 40,000, the Shoulds become the adjustment variable. It is the tool that kills the classic "everything is a priority".
| MoSCoW priority | Target share of backlog | Budget treatment |
|---|---|---|
| Must have | 50-60 % | Fixed, committed scope |
| Should have | 20-25 % | Delivered if budget holds |
| Could have | 15-20 % | v2 backlog |
| Won't have (now) | remainder | Documented, excluded |
For estimation, a complexity-points grid (1, 2, 3, 5, 8, 13) applied to stories, crossed with a day rate of 450 to 750 EUR HT at a London agency, gives a defensible range. A 5-point story is usually worth 2 to 4 person-days.
| Complexity (points) | Indicative person-days | Cost at 600 EUR rate |
|---|---|---|
| 1 (trivial) | 0.5 | 300 EUR HT |
| 3 (simple) | 1.5 | 900 EUR HT |
| 5 (medium) | 3 | 1,800 EUR HT |
| 8 (complex) | 5 | 3,000 EUR HT |
| 13 (very complex) | 8-10 | 4,800-6,000 EUR HT |
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Mini case study
Mark, operations director of a logistics SME in London (95 staff), wants a route-tracking app. Without a spec, his first three quotes range from 38,000 to 110,000 EUR HT: impossible to compare. He writes a 28-page spec with 46 user stories, including 26 Must (140 points), 12 Should (55 points), 8 Could.
The Musts, estimated at 84 person-days x 600 EUR = 50,400 EUR HT. He sets the envelope at 55,000 EUR HT, keeps the Shoulds as a variable and excludes the Coulds. Result: the three new quotes land in a 52,000 to 61,000 EUR HT range, a 17 % spread instead of 190 %. The spec saved him weeks of arbitration and secured acceptance.
FAQ
Do you need a spec for a simple MVP? Yes, but a lean one: 8 to 15 pages with 15 to 25 user stories is enough. Even a 30,000 EUR HT MVP deserves written acceptance criteria to avoid misunderstandings.
Who should write the requirements spec? Ideally the business side helped by a business analyst. Budget 4,000 to 9,000 EUR HT for external scoping, often repaid at the very first budget trade-off.
Does the spec freeze the project? No. In agile it fixes the vision and the Musts, but the backlog stays alive. Acceptance criteria and penalties, however, are contractual and do not move.
How do I include late-delivery penalties? Plan 0.5 to 1 % of the lot amount per week of delay, capped at 10 %. They should target milestones, not each story.
How does it differ from a technical spec? The functional spec describes the need; the technical spec (often produced by the vendor) describes the architecture. Leave the how to the vendor so you can compare approaches.
Let's scope your project. Send us your business need and the rough number of user journeys: we help you structure a MoSCoW-prioritised spec with an estimation grid. 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.
