The agency-vs-in-house debate is usually framed as a cost comparison. It isn't. It's a control comparison — and the costs only make sense once you understand what each model actually buys you.
01The true cost of an agency retainer
A £6,000-a-month agency retainer rarely costs £72,000 a year. It costs that plus the senior internal time spent managing the relationship, briefing campaigns, chasing reports, mediating between the agency and your sales team, and re-pitching the work internally every renewal cycle.
For most £1M–£10M businesses, that overhead adds another £30,000 to £50,000 in genuine cost. The retainer is the visible line; the management drag is the invisible one.
02The true cost of in-house
A £55,000 marketing manager is rarely £55,000. Add 20% employer's NI, pension, equipment, software stack, recruitment fees and ramp-up time and you are at £80,000–£90,000 fully loaded, before they have produced a single result.
Worse: a mid-level in-house hire still needs senior strategic direction. If the founder is providing that themselves, that's another 5–10 hours a week the business doesn't see on a P&L.
03The hybrid that usually wins
The model that fits most £1M–£20M businesses is fractional senior leadership plus one or two specialist agencies plus a small internal coordinator. The fractional CMO owns the strategy and accountability. The agencies execute paid and creative. The coordinator keeps the wheels turning day to day.
Total cost typically lands between an agency retainer and a full in-house team — with the strategic discipline of neither alone.