Why the question scares — and deserves real numbers

When "custom web app" comes up, the first concern is budgetary. And it's legitimate: vendor estimates range from €8k to €800k for projects that sound the same. This article demystifies a budget's composition and gives realistic ranges by typology.

Important — These ranges are indicative, based on real projects. Precise scoping requires a conversation: depending on your exact scope, you may be at the low end, high end, or even outside the range.

The 5 cost categories

1. Scoping (typically 5 to 15% of total budget)

Before coding, you need to know what to build. Scoping covers:

  • User and business needs interviews
  • Functional perimeter definition (what we do, what we don't)
  • Data modeling (entities, relations, rules)
  • Definition of key screens (wireframes)
  • Planning and milestones estimation

On a small project, scoping can take 2-3 days (€1,500 - €4,500). On a complex project, it can stretch over 3-4 weeks (€8-15k).

Skipped or sloppy scoping is the most expensive mistake on a web project. Re-doing a poorly thought-out screen costs 3-5 times more than thinking it through at the start.

2. Design and UX (10 to 25% of budget)

Not just "decoration". The design of a web app covers:

  • Information architecture (how the user navigates)
  • Design of key screens (with finalized visuals)
  • Design system (reusable components, consistency)
  • Responsive behavior (mobile / tablet / desktop)
  • Accessibility (WCAG, contrasts, keyboard navigation)

On Panorama Pub, design represented about 15% of the budget. On more complex projects (rich user area, multiple roles, dashboards), it easily reaches 25%.

3. Development (50 to 70% of budget)

It's the main category. It covers:

  • Front: development of all screens (Next.js, TypeScript, Tailwind)
  • Back / API: business logic development
  • Database: schema modeling and implementation
  • Admin: management interface development
  • Authentication: accounts, sessions, roles
  • Integrations: connections to third-party services (Stripe, SendGrid, etc.)
  • Tests: unit, integration, end-to-end
  • CI/CD: automated deployment pipeline

At a daily rate of €450 HT (Balance tier), the development of a simple web app represents 15-25 days (€7-11k). For a rich marketplace or business tool, you land on 40-80 days (€18-36k).

4. Infrastructure and hosting (5 to 10% of initial budget, then recurring)

  • Vercel hosting setup
  • Serverless PostgreSQL database configuration (Neon, Supabase, Vercel Postgres)
  • Domain, SSL, CDN configuration
  • Monitoring setup (Sentry, Vercel Analytics, etc.)
  • Backups and security configuration

Recurring: €20 to €200/month depending on traffic and database size.

5. Maintenance and evolution (15 to 30% of initial budget per year)

Over the 12 months following launch, you typically observe:

  • Fixes: bugs reported by users (5 to 10% of initial budget)
  • Small evolutions: UX adjustments, new labels, small functional additions (10 to 15%)
  • Major evolutions: significant new features (per the roadmap)

For a web app at €30k initial, plan €6-9k of maintenance/evolution in year 1, then 15-20% per year thereafter.

Ranges by typology

Simple web app (internal tool, configurator, simulator)

  • 1 to 2 user roles
  • 3 to 5 business entities
  • 10 to 15 main screens
  • No complex third-party integration

Initial budget: €12 to €25k Lead time: 6 to 10 weeks

Medium web app (simple marketplace, business platform, client area)

  • 2 to 4 user roles
  • 5 to 10 business entities
  • 20 to 40 main screens
  • Standard integrations (payment, transactional email)
  • Rich admin

Initial budget: €25 to €50k Lead time: 8 to 16 weeks (Panorama Pub is in this range: ~€30k, 8 weeks)

Complex web app (mature marketplace, B2B SaaS, events platform)

  • 4+ user roles with fine-grained permissions
  • 10+ business entities with complex relations
  • 50+ screens
  • Multiple integrations (CRM, ERP, external services)
  • Analytics dashboards
  • Elaborate business workflows

Initial budget: €50 to €150k Lead time: 4 to 9 months

Mobile application (PWA)

  • Often a sub-scope of a web app, designed for mobile
  • Geolocation, local persistence, offline mode as needed
  • No mandatory in-app payment (otherwise native)

Initial budget: €8 to €30k (standalone) Lead time: 4 to 10 weeks

The OETH benefit — 30% reduction

If you're a French company subject to the disabled worker employment obligation (OETH), you can deduct 30% of the labor cost from your annual AGEFIPH contribution, within applicable limits.

Concretely, on a €30k HT project with €25k of labor, you can deduct up to €7,500 from your AGEFIPH contribution. Net project cost drops to €22.5k.

Conditions: your vendor must be a TIH (French independent worker with disability) — that's my status. Learn more about the OETH benefit.

Traps that explode a budget

1. Silent scope creep

"Could we also add a notification system? And while we're at it, user badges. And an admin page to manage that." Each mini-addition mid-project costs 2-4 times more than if it had been planned at scoping.

Mitigation: solid scoping, and a clear protocol for out-of-scope evolutions (separate quotes).

2. The "like [big player]"

"We want something like Airbnb / Doctolib / Le Bon Coin." These platforms cost tens (hundreds) of millions to build over 5-10 years. Wanting their equivalent in V1 guarantees failure.

Mitigation: MVP focus. What is the central feature that creates 80% of the value? We do it well. The rest, later.

3. The endless design phase

More than 3 design iterations → you're looking for something other than design (often: you don't yet know exactly what you want to build). Back to scoping.

Mitigation: a fixed number of design iterations, contractual.

4. Underestimated integrations

"You just integrate with our CRM, should be quick." Except the CRM has an outdated API, poorly documented, and needs 3 days just to stabilize.

Mitigation: audit each integration at scoping. Plan a 30-50% margin on integrations flagged as risky.

5. Unbudgeted maintenance

At launch, all is well. Three months later, a critical bug emerges, and the budget is gone. Project dies.

Mitigation: budget 15-25% of initial cost in year-1 maintenance from the start.

What does NOT save money

  • "Can we do half in WordPress and half in web app?" → No, having 2 stacks costs more.
  • "Can we copy-paste a similar web app?" → No, business specificity is precisely what costs money to model and code.
  • "Can we have the design done by a cheaper freelancer in parallel?" → Rarely viable. Design and development are built together.

How I bill, concretely

For most projects under €20k, I propose a fixed price: a set amount for a set scope. Client advantage: no surprise. Developer advantage: no uncovered overrun.

Beyond that, I work on a daily rate (between €350 and €650 HT/day depending on the project), with precise scoping and validation points at each milestone. It's more flexible — and requires rigorous scoping to avoid scope creep.

Going further

For a personalized estimate on your project: run the diagnostic. Reply within 24h, quote within 48h.