Launching a website without a specification document (CDC) often leads to doubled costs and major delays. This document turns your ideas into a clear plan, limits misunderstandings with developers, and acts as a “contract” between you and the agency: it specifies what you want, why, and how to achieve it. Without a CDC, a large share of projects exceed budget and deadlines (up to 70% according to studies), whereas a well-structured CDC significantly reduces risks (around 50%) by framing expectations, deliverables, and scope.
1) Laying the strategic foundations: why, for whom, and what makes you different
Start by defining SMART objectives (Specific, Measurable, Achievable, Realistic, Time-bound), for example: “increase online sales by 20% in six months.” Link these objectives to KPIs: in e-commerce, conversion rate (often 2–5% initially) and time spent on page (ideally > 2 minutes) help measure engagement. The key idea: connect each objective to a concrete feature (e.g., lead generation = a simple, visible signup form).
Next, build the site for your users through personas (3 to 5 typical profiles based on real customers), segmented by age, needs, and digital habits. A persona like “Marie, 35, active mother looking for flexible courses” directly influences UX.
→ simple navigation, mobile experience, fast journeys. A user-centered site can improve loyalty (up to +30% according to web statistics). Use existing data (surveys, customer feedback) to validate these profiles.
Finally, conduct a competitive analysis on 2–3 close players: structure, navigation, mobile performance, content, SEO. Identify their strengths and weaknesses (e.g., strong SEO but…).
…confusing UX, slow site, poor mobile optimization), then formalize your USP (unique selling proposition), such as “free delivery within 24h.” This USP must appear clearly on the homepage through concrete elements (e.g., price calculator, service promise, proof).
2) Defining the functional scope: what the site must do
The CDC must list features, distinguishing must-have (MVP to launch quickly) and nice-to-have (next phase). Example showcase site: must-have = services, about, contact pages; nice-to-have = live chat, blog. Example e-commerce: secure payment (Stripe) as must-have. This prioritization protects budget and schedule: the MVP allows quick testing with real users before adding options.
Then structure the information architecture via a sitemap (page tree) and user journeys (e.g., homepage > product > cart > payment), aiming for key actions accessible in under 3 clicks. Clear navigation can reduce bounce rate (up to -40%). Also describe typical steps: arriving via Google, exploring categories, converting through explicit call-to-actions.
Add technical specifications and integrations: CRM (HubSpot), analytics (Google Analytics), email marketing (Mailchimp), payment methods (PayPal, credit card), CMS choice (WordPress for simplicity, custom if complex needs), and required APIs.
Also anticipate integration costs: late or unsuitable choices increase the bill.
3) Non-functional requirements: performance, security, SEO, compliance
On performance, require loading within 3 seconds and alignment with Core Web Vitals: a slow site can drive away up to 50% of visitors. On security, enforce SSL, DDoS protection, and reliable hosting (e.g., OVH). Frame maintenance (monthly updates, e.g., €100/month) and post-launch support (bug management, service levels).
SEO must be integrated from the CDC: H1–H6 tags, meta descriptions, clean URLs (e.g., /product/simple-name), 301 redirects in case of migration, responsive design (with ~60% of traffic on mobile), and tools like Yoast on WordPress. An SEO-ready site can multiply organic traffic (up to +200% in one year).
Finally, ensure legal compliance: GDPR (cookie consent, privacy policy, legal notices). Also add accessibility per WCAG (contrast, alt text), mandatory for some public sites and beneficial for all. Tools like WAVE help check, and GDPR non-compliance can lead to fines up to 4% of turnover.
Key takeaway: define 3 SMART objectives + KPIs, create 3 personas, analyze 2 competitors and formalize the USP, draft a first CDC then validate it with a professional before approaching agencies. With a ready CDC, you secure budget, deadlines, and results.