Build or buy: when your own tool wins

All Stack Labs, 12 October 2026

Most of the software a business runs should be bought. Email, payroll and accounting are solved problems, and building your own would waste money. But the stack keeps growing. Zylo, which sells SaaS management software, put median SaaS spend at $9,455 per employee in its 2026 index, and found companies leave 36% of their licences unused. In its 2025 index, companies with up to 500 employees ran 152 apps on average.

Somewhere in that pile is a tool your team bends its work around, or a spreadsheet doing a job no tool covers. That is where building pays.

Renting keeps getting more expensive

Subscriptions do not hold still. Vertice, another spend management vendor, put SaaS inflation at 16.4% in June 2026, the highest on its index. The biggest vendors are also adding AI to their plans and charging for it:

Microsoft raised Microsoft 365 prices from July 2026. Business Standard went from $12.50 to $14 per user a month.
Google added Gemini to Workspace in 2025 and moved Business Standard from $12 to $14.
Salesforce raised Enterprise and Unlimited list prices by an average of 6% in August 2025, and sells its AI agents on top, as add-ons or at $0.10 per action.

Per-seat pricing also taxes growth. Each new hire adds a seat on each per-seat tool.

Your spreadsheet is already a tool you built

When no product fits, the work lands in a spreadsheet. Spreadsheets suit analysis but are fragile as systems. Raymond Panko's review of the research put the average error rate at 3.9% of cells in lab studies, and detailed field audits found errors in 94% of the spreadsheets they examined. His other finding matters more: the people who build spreadsheets are highly overconfident that theirs are right.

Two well-known failures show the risk. In October 2020, Public Health England left 15,841 positive COVID cases out of its daily figures, which press reports traced to lab files loaded into an old Excel format with a row limit. And in January 2012, a JPMorgan model review described the risk calculation behind the trades later known as the London Whale as "done off spreadsheets using a manual process" and "error prone". The US Senate published it the following year.

If a spreadsheet moves data between systems, depends on copying numbers by hand or holds the only record of something important, it is already an internal tool. It just has no tests, no permissions and no history.

When to build

Martin Fowler's test still holds. Ask whether the function is a differentiator. If it is a utility, buy the package and adjust your process to fit it. If it is part of what makes you better than your competitors, you do not want the same software they have. Build when:

It is how you win. Quoting, scheduling or a client portal that customers choose you for.
You pay for a platform to use a few screens of it. Each price rise lands on every seat.
A spreadsheet is doing integration work. As Thoughtworks' Brandon Byars put it, you can't buy integration. If people copy data between systems, that job belongs in software with logs and alerts.
Your data or your rules are unusual. Regulated records, odd pricing, or approvals no product models well.

Buy when the function is a commodity, a good product exists and its roadmap is one you want.

The real cost of building

A tool is not finished at launch. A widely cited estimate from Robert Glass's Facts and Fallacies of Software Engineering puts maintenance at 40 to 80 percent of software costs, with 60 percent the average. Plan for it from the first day.

Build it so that work stays cheap: a standard stack, tests, documentation and everything in your own accounts. Then any capable engineer can extend it, and you are never tied to the people who wrote it. That is how we build internal tools, and why we hand them over.

The rule

Buy the commodities. Build the part that makes you different, and the part your spreadsheets are holding together. When you compare costs, count the seats you will add over the next three years, not the ones you pay for today.

Build or buy: when your own tool wins.