SEM for ecommerce: what a good agency should do for your store (UK guide)

If you sell online, search engine marketing (SEM) is usually the fastest way to put your products in front of people who are ready to buy. But handing your Google Ads account to an agency is a big decision. This guide explains what a good SEM agency should actually do for an ecommerce store, how to judge whether it is working, and what to ask before you sign anything.

What SEM means for an online shop

SEM is paid advertising on search engines, which in the UK mostly means Google Ads. For a lead-generation business, SEM is about keywords and enquiry forms. For an ecommerce store it is different: most of your sales will come from product-based campaigns that show your products, prices and images directly in the search results.

That changes what good management looks like. The best ecommerce SEM work happens in your product data, your tracking and your margins, not just in keyword lists.

The campaigns that matter for ecommerce

• Google Shopping: product listings with an image, price and store name. Driven by your product feed in Google Merchant Center.
• Performance Max: Google’s automated campaigns that run across Search, Shopping, YouTube, Display and Gmail. Powerful, but they need good data and sensible structure to avoid wasting budget.
• Search: text ads for your brand name and your most valuable category searches.
• Remarketing: ads that bring back people who viewed products or left items in their basket.

Most stores do best with a mix: Shopping or Performance Max for product sales, brand Search to protect your name, and remarketing to recover nearly-buyers.

Why your product feed matters more than your bids

Google decides which searches to show your Shopping ads for largely from your product feed. Weak titles, missing attributes and disapproved products quietly limit your reach, and no amount of bidding fixes that.

A good agency will look at your feed before touching budgets. That means clear product titles that use the words shoppers search for, complete attributes such as brand, colour, size and GTIN, good images, and fixing Merchant Center errors.

Tracking: the foundation everything else depends on

Google’s bidding uses your conversion data to decide where to spend. If tracking is broken or incomplete, Google optimises on the wrong signals. Before scaling any campaign, check that:

• Purchases and revenue are tracked accurately in Google Ads and GA4.
• Enhanced conversions are set up, which helps recover sales that cookies alone miss.
• Consent mode is configured correctly. For UK and EU visitors, Google requires consent signals to be passed for its measurement and remarketing features to work fully.

Setting the right target: ROAS and margin

Return on ad spend (ROAS) is revenue divided by ad spend. A ROAS of 4x means £4 of revenue for every £1 spent. But the number that matters is whether that revenue is profitable once you account for your margin.

A simple way to find your break-even point is to divide 1 by your gross margin. If your gross margin is 40%, your break-even ROAS is 1 ÷ 0.4 = 2.5x. Below that, ads lose money on the first sale. Above it, they are profitable. A good agency will agree a target with you based on your margins and how much you value repeat customers, not chase a generic number.

How to choose a SEM agency for your store

• You own your accounts. Your Google Ads, Merchant Center and GA4 accounts should be in your name, with the agency given access. If you part ways, you keep everything.
• Ecommerce experience. Ask for examples of Shopping and Performance Max work, not just lead-generation campaigns.
• Clear fees. Ad spend should go directly to Google from your account. The management fee should be separate and easy to understand.
• Reporting you can read. You should get a regular report covering spend, revenue, ROAS and what is being changed next, plus time to talk it through.
• Reasonable terms. Campaigns need time to learn, so a short initial term is normal. Long lock-in contracts are a warning sign.

Questions to ask before you sign

1. What will you look at first in my account, and why?
2. How will you structure my campaigns around margin and best-sellers?
3. What will you check in my product feed and tracking?
4. What target ROAS or cost per sale do you suggest, and how did you work it out?
5. What will I receive each month, and who will I speak to?