Handing your tracking to an agency is easy. Your specialist moves on, the agency already runs your paid media, and giving them Tag Manager too feels like one less thing to manage. The cost shows up later, and it's usually bigger than the fee.
Agencies aren't the problem. Plenty do good work. The problem is an arrangement where the same people build the tracking, run the ads it measures and report on the results, and nobody on your side checks any of it.
What it costs.
Numbers you can't trust.
When the team running your ads also decides what counts as a conversion, and what it's worth, the numbers tend to flatter the ads. A form fill counted as a sale, a page view sent twice, a key event that fires on every visit.
Often nobody chose that on purpose. But once the board has seen a trend, it's hard to admit the trend was never real.
Ad spend steered by the wrong signal.
Google Ads and Meta optimise towards whatever you tell them is valuable. If that signal is inflated or mislabelled, the platforms spend your budget finding more of the wrong thing, and the reports say it's working.
Consent that isn't what your banner says.
Each campaign adds a pixel. If nobody checks each one against your consent rules, some will fire before a visitor has chosen, or after they've said no. Your privacy notice becomes untrue, and it's your business the ICO writes to, not the agency.
A slower website.
Every tag is more JavaScript on every page. Containers that only grow end up with dozens of scripts nobody remembers adding, and mobile visitors pay for it in load time and responsiveness.
Lock-in.
If the accounts sit under the agency's name and the only documentation is in their heads, leaving becomes expensive. You either stay, or you pay someone to rebuild what you already paid for.
The rescue bill.
Problems left for a year or two don't stay small. Reports have to be rebuilt, historical data can't be corrected, and the clean-up costs far more than the checks that would have prevented it.
Why it happens.
None of this needs bad intent. It's what the arrangement rewards.
- Adding is the job, removing isn't. Every campaign adds a tag. Nobody is paid to take them out, so containers only grow.
- Nobody checks the release. If the people publishing changes are the only people who understand them, mistakes go live and stay live.
- The agency marks its own homework. Measurement and media sit with the same team, so there's no independent view of whether the numbers are right.
- Knowledge leaves with people. Account teams change. Without documentation, each new person works around what they find instead of fixing it.
- Consent is nobody's job. The banner belongs to legal, the tags belong to the agency, and the gap between them belongs to no one.
Keep the keys.
You can still use an agency for the work. What matters is that your business owns the setup:
- Check the claims before you hire. Ask for named examples of similar work and speak to those clients. Better still, give a small paid task on your own setup first.
- Own the accounts. Tag Manager, GA4 and every ad account should belong to your organisation, with the agency added as users. If they leave, the accounts stay.
- Review every release. Someone on your side, or someone independent, looks at each Tag Manager version before it's published. Limit who can publish, and who can add custom code.
- Write the rules down first. A measurement plan and a dataLayer specification that your developers build and the agency works to, not the other way round.
- Define your own conversions. What counts as a lead or a sale, and what it's worth, is a business decision. Don't leave it to the people optimising towards it.
- Test consent on every release. A private window, banner untouched, then "Reject all" and a reload. It takes five minutes, and the guide to testing Consent Mode v2 shows what to look for.
- Make removal part of the job. Every tag has an owner and a reason. Anything without one comes out at the quarterly review.
- Ask for a handover document. Whenever anyone leaves, agency or staff, they leave behind what's live and why.
Where an independent check fits.
The simplest protection is to separate doing the work from checking it. The agency builds; someone with nothing to sell you in media checks.
That's the role Marc Alexander usually plays alongside agencies: an audit to find out where things stand, then monitoring that catches problems after each release. If you're weighing up who should do what, choosing an analytics consultant or an agency sets out the trade-offs, and three red flags covers the advice to be wary of.
If you don't know what's in your container today, the GA4 mini audit is the quickest way to find out.