The most expensive mistake: building what already exists
One of the most common mistakes we run into when we take on a new project is an internal webapp that reinvents features available in any SaaS for €30 a month: user authentication, permission management, email notifications, PDF generation, invoicing.
Each of those takes days to build and weeks to debug. A company that builds its own invoicing system when it could use Stripe, Holded or Factura Directa is spending between €20,000 and €60,000 on something that sets it apart from precisely nobody.
When it makes sense to build custom
Building makes sense when the process you are automating is the source of your competitive advantage. If your order assignment algorithm, your data analysis method or the way you work with clients is what sets you apart, a standard SaaS will either hold you back or push you into working the way the vendor thinks you should.
It also makes sense to build when the volume of use makes the SaaS bill unsustainable. A company with 500 active users can pay €3,000 a month in SaaS licenses that its own webapp would cover for €500 a month in infrastructure.
Total cost of ownership (TCO) analysis over 3 years
The fair comparison is not 'development cost' against 'monthly SaaS price'. It is the total cost of ownership over three years:
- SaaS: (monthly price × 36) + customization cost + integrations + data migration + vendor lock-in risk.
- Custom: initial development + hosting/infrastructure × 36 + maintenance + security updates + team cost for product evolution.
- In most of the cases we analyze, the custom solution starts working out cheaper than the SaaS somewhere between month 18 and month 30.
The hybrid model: build on top of SaaS APIs
In most cases the smartest strategy is neither 'buy everything' nor 'build everything'. It is building the business logic that is genuinely yours on top of SaaS APIs for the standard pieces: Stripe for payments, Supabase for authentication and the database, SendGrid for email, Mux for video, Twilio for SMS.
That way you keep full control of the experience and the business logic while handing commodity infrastructure to providers who maintain and scale it better than any small internal team could.
Frequently asked questions
Maintaining a well-built webapp costs between 15% and 25% of the development cost per year. A webapp that cost €40,000 should be maintainable for €6,000 to €10,000 a year, covering security updates, dependencies and small improvements. If maintenance goes over 30%, it is a sign of technical debt or a poor initial design.
An internal webapp that automates one specific process takes between 8 and 16 weeks. A platform with client access, payments and an admin panel takes between 16 and 28 weeks. Both timelines assume a team of two or three people and a scope agreed before work starts.
Data migration is the biggest hidden cost of moving from SaaS to custom development. Before you commit to any SaaS, check two things: can I export my data as CSV or JSON whenever I want, and is there an API that reaches all of it? If the answer to both is no, vendor lock-in should weigh heavily in the decision.