You should not need to. PPC results show within weeks, so a rolling monthly agreement with a short notice period is fair to both sides. A 12-month minimum term mainly protects the agency from losing a client who is not happy. Scalepoint works month to month with 14 days’ notice and no exit fees.
If an agency insists on a minimum term, ask what you get in return, such as a lower fee or work you would otherwise pay for. Then check the exit terms: the notice period, any fees for leaving early and how quickly they hand over access.
Whatever the contract says, keep the ad account in your name. That way a contract dispute never puts your campaigns or data at risk.
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This is one part of a bigger picture. How to choose a PPC agency in the UK covers costs, pricing models, ownership, contracts, tracking and red flags in one place.