Case Study: How Consent Management Platforms & First-Party Data Increased Google Ads ROAS by 263%
If you’re a business owner and your website doesn’t have a properly installed Consent Management Platform (also called a cookie banner or CMP), your Google Ads will undoubtedly begin to suffer. This has become even more prevalent in the last few months in the wake of new changes to privacy laws.
In response to this, the Third Marble team built a proprietary “Data Chain Process” to solve this exact problem. Here’s how we used it to turn things around for a national home fixture retailer, taking their ROAS (return on ad spend) up by 263% and boosting total leads by a whopping 605%.
Campaign Snapshot
| Metric | Baseline (Apr 7 – May 6) | Peak (Jul 6 – Jul 28) | Overall Change |
|---|---|---|---|
| Conversions (Leads) | 9.00 | 63.45 | +605% |
| ROAS (Conv. Value / Cost) | 1.34 | 4.86 | +263% |
| Cost / Conv. (CPA) | $390.00 | $94.19 | -75.8% |
The Impact of CMP Installation
Before setting up proper tracking, this retailer struggled with high acquisition costs and low volume. They were generating only 9 conversions for $390 per conversion and a ROAS of 1.34. On May 1st, 2026, we installed Enhanced Conversion Tracking alongside a compliant CMP integrated with Consent Mode v2.

Within a week of proper CMP installation and Enhanced Conversion Tracking, performance significantly improved. With clear signals feeding Google's Smart Bidding algorithms, lead volume doubled over the next month to 19.04 conversions, while cost per lead dropped to $316 and ROAS climbed to 1.49.

A Break in the Data Chain: What Happens When a CMP Is Removed?
On May 25th, 2026, the client removed their CMP, which caused a significant break in the Data Chain. Consequently, their results immediately took a downturn. Without consent signals, Google's algorithms lost visibility into who was converting. Cost per conversion spiked from $316 up to $889, while ROAS crashed to 0.29 and total conversions fell to 3.57.

On June 10th, 2026, the client reinstalled the CMP, and again, within a week, their results more than doubled. Conversions jumped back to 10.38, their cost per lead fell back down to $106, and ROAS recovered to 3.55.

Adding to the Data Chain
With the CMP and Enhanced Conversion Tracking working smoothly, we built onto the Data Chain. In early June, we implemented Customer Match Lists to give Google’s machine learning models clear target audience profiles.

Over the course of 60 days, results continued to improve as the data chain became complete. By late July, campaign performance hit record highs: leads scaled to 63.45, cost per conversion reached a low of $94.19, and overall ROAS topped out at 4.86.

What are the Key Takeaways for Advertisers?
- CMP Infrastructure Drives Smart Bidding: A Consent Management Platform isn't just a legal requirement for privacy compliance. It’s a critical signal loop for Google's Smart Bidding algorithms.
- Tracking Gaps Cause Instant Performance Drops: Removing consent tracking causes Google's machine learning models to lose conversion visibility within days, leading to inflated CPAs and wasted ad spend.
- First-Party Data Multiplies ROAS: Combining Google Consent Mode v2, Enhanced Conversions, and first-party Customer Match lists builds a complete "Data Chain" that systematically lowers Customer Acquisition Costs (CAC).
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