Below this range, the volume of routine work (helpdesk tickets, patching, device provisioning) rarely justifies a full-time hire, and a single IT generalist covering everything alone has no backup when they are on leave, or when they leave the company entirely. A managed provider brings a team rather than a person, defined SLAs rather than best-effort availability, and continuity that does not depend on one individual. The tradeoff is less day-to-day proximity and a support model that has to be managed through a contract rather than a conversation.

As headcount grows, organisations typically bring on internal IT staff for the parts of the role that benefit from proximity and business context: vendor relationships, change approval, and systems specific to the business. A co-managed arrangement then covers the parts that scale better externally: monitoring, patching, after-hours coverage, and specialist security work that an internal generalist would otherwise need to learn from scratch. The critical design decision in a co-managed model is an explicit, written split of who owns what. Ambiguity about who is watching a given system is how incidents go unhandled, each side assuming the other has it.

Organisations with an established internal IT department rarely need full outsourcing, but frequently still buy managed services for specific, hard-to-staff functions: 24/7 monitoring coverage that an in-house team of standard working hours cannot provide alone, specialist security operations, or overflow capacity during a major project or migration. The decision here is narrower and more tactical than the earlier stages.

Whichever model you land on, in-house, fully managed, or co-managed, three things are worth getting in writing from day one: a defined SLA for support response, a documented plan for what happens to access and data if the arrangement ends, and clarity on who is accountable when something breaks at 2am. Organisations that skip this step tend to discover the gaps only during an actual incident, which is the most expensive time to find them.