Custom web app & MVP

When no software on the market fits your business, we build your custom web app.

Marketplace, vertical B2B SaaS, client portal, back-office: in production in 4 to 6 weeks, built around your process. If an off-the-shelf tool covers your need, I will recommend it. Sometimes the right answer is to build nothing.

What I build

What changes: one tool, built around your process.

Four project families come up again and again. Every time, the same principle: start from your real process, ship a usable product, improve after real use.

Most requested

Marketplace MVP

Supply meets demand with payments built in: sign-up on both sides, profiles, matching, commissions or automatic payouts.

  • Automated commissions and payouts
  • Seller and buyer onboarding
  • Matching, reviews, notifications
  • Moderation back-office included
Best for

testing a two-sided business with real paying users, before investing further.

Discuss this project

Vertical B2B SaaS & internal tool

The logic of your process, not a generalist vendor's. Multi-tenant accounts, roles, subscriptions, dashboards.

  • Multi-organisation accounts and roles
  • Built-in subscriptions and billing
  • Import and export of your data
  • Business dashboards
Best for

SMEs whose process runs on Excel and five badly connected SaaS, or founders of a niche SaaS.

Discuss this project

Client portal & back-office

Everything your clients or your teams need to see and do, in one place: case tracking, documents, approvals, automatic follow-ups instead of emails and attachments.

  • Secure client area
  • Approval workflows
  • Document generation
  • Connected to your existing tools
Best for

firms, agencies and service operators who run their clients through email and shared spreadsheets.

Discuss this project

Community platform

Members, content, events, messaging, moderation: your community on your own turf, with your rules, rather than on a Facebook group or a free Slack.

  • Profiles, memberships, payments
  • Content and events
  • Email and in-app notifications
  • Moderation and roles
Best for

associations, clubs, professional networks, paid communities that want to own their audience.

Discuss this project

Recently shipped, anonymised: a B2B monitoring SaaS designed solo and now in production, community platform work, several business back-offices. The starting point is often the same: spreadsheets and tools that do not talk to each other. The end point: a single workspace, used every day. Your project is not on the list? We scope it in 30 minutes.

How a project works

A tool in production, not a six-month project.

We slice the scope down to the version that does the job, ship it to production in 4 to 6 weeks, then improve based on real-world use rather than assumptions.

01

30-minute scoping call

We start from your process and the expected outcome, not a 40-page spec. You leave with a working product perimeter, a timeline and a written quote.

02

Built in 4 to 6 weeks

Main flow, accounts, payments if needed, back-office. You watch the product move forward every week on a staging URL.

03

Production deployment

Deployment, monitoring, backups. You receive the handoff documentation and admin access.

04

Iterate after real use

Real users decide what is missing. We prioritise and ship in small increments, through a monthly retainer if you want a steady rhythm.

Framework & trust

Your product, your keys.

A custom application is only useful if it holds up over time. Here is the framework, the same for every project.

01

You own the code

A Git repository you own, admin access, clear documentation. Everything is transferable to another team whenever you decide, with no hidden lock.

02

GDPR & France / EU hosting

Data exportable at any time, compliance considered from the scoping call onwards.

03

One single point of contact

The person who scopes is the one who codes, deploys and maintains. No subcontracting. NDA signed on request before any detailed discussion.

04

Serious about production

Deployment with monitoring and backups, handoff doc delivered with the product. Maintenance and evolutions through a monthly retainer, at your pace.

FAQ

Questions I get about these projects.

How much does an MVP cost?

The price depends on scope, framed in a 30-minute call. The public anchors: a Sprint from €2,500 for a small, well-defined scope; a full 4-to-6-week MVP on quote; a monthly retainer on quote for evolutions. The quote is written before anything starts.

How fast is my application delivered?

A genuinely usable product goes to production in 4 to 6 weeks in most cases: main flow, user accounts, payments if needed, back-office. A small, well-defined scope fits in a 1-week Sprint. You watch progress every week on a staging URL.

Who owns the code?

Fully yours: a Git repository in your name, admin access, handoff documentation. Everything is transferable to another team at any time, with no hidden dependencies and no proprietary licence.

What tools do you use, and why?

Proven, well-documented building blocks: Next.js for the application, Supabase for data and authentication, Stripe for payments. Standard choices that any developer can pick up. I also rely on modern tooling to deliver faster.

What if I already have developers?

No problem: I join as an additional developer on a well-defined scope, in your repository or a dedicated one, with code reviews and clear documentation. Your team takes over without friction; that handoff is planned from day one.

What happens after launch?

The application ships to production with monitoring and backups, and you receive handoff documentation. Then two options: your team takes over, or a monthly retainer covers maintenance and evolutions.

Shall we scope your project?

Describe the product or tool you have in mind. In 30 minutes, I tell you what a working product contains, how fast it ships, and at what price.

Book a call Chat with my AI agent Direct email

Mathieu Haye, a freelance developer specialised in AI and automation based in Paris, designs and ships custom web applications and MVPs to production in 4 to 6 weeks: marketplaces, vertical B2B SaaS, internal business tools for SMEs, client portals, back-offices and community platforms. The approach starts from your processes: a genuinely usable product first, then improvements decided on real-world use.

Custom app or off-the-shelf SaaS?

If an off-the-shelf SaaS covers your need for a reasonable subscription, buy it: custom development is not justified. It becomes rational when several subscriptions, exports and double data entry stand in for a process. A custom business application encodes your exact process and replaces that pile. A middle case: if your current tools are fine but do not talk to each other, an n8n automation is often enough, with no app to build. The 30-minute scoping call exists to pick the right option, including when the right answer is to build nothing. Formats and price anchors are detailed on the collaboration page; delivered work is shown on the projects page.

What if the need goes beyond the app?

Looking for a CRM shaped around your sales cycle? That is a neighbouring but distinct job, told on the custom CRM page. If your product needs an embedded AI agent (customer answers, document search), the AI agent developer page covers those cases; for the full picture of services, see the AI freelance page.

Have a product or a tool in mind? Describe it to my AI agent or go through the collaboration page to book a scoping call. See also: the AI agent for SMEs, your AI maturity, AI visibility (GEO), the homepage and the journal on applied AI.

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