Buying software is usually the right first move. But there's a point where SaaS starts working against you.
Buy when the problem is common
If your need is standard and a mature tool fits, buy it. Don't build a calendar.
Build when the process is your edge
When your workflow is a competitive advantage — a unique sales pipeline, a specialized operations flow — forcing it into someone else's product creates friction, workarounds, and shadow spreadsheets.
The tipping point
Build when you're paying per-seat for software your team fights, integrating five tools that should be one, or unable to get the data you need. Custom software that fits your process pays for itself in adoption and clarity.