There’s more than one way to create a mobile commerce web site, and Google Inc. now has something to say about it. In a recent blog post, the search kingpin and Android maker offers three recommendations when it comes to creating a mobile presence that Google can easily crawl and index for consumers searching the mobile web. And its No. 1 suggestion is responsive web design.

Responsive web design uses one set of content to build web sites on the fly that best fit the screen size of the device requesting the site. It’s somewhat modular, shifting blocks of content up and down or left and right to create a site that looks good on a PC, a tablet, a smartphone or even a TV. Responsive design saves a retailer from having to build separate sites optimized for smartphones and tablets. But responsive design is resource-intensive, and requires coding skills that are not yet common.

“The web is ever expanding and for Google that means adding all of this processing power to crawl and index the web,” says Katerina Potter, search marketing specialist at iProspect, a search engine marketing firm. “When you have all these different devices and a separate site for each, that eats up a lot of processing power on the part of the search engine, so Google is thinking ahead and making sure people think about reusing resources in a singular way.”

Responsive design gives a retailer one set of content on one URL that can be used across devices. “In the long term it’s a win-win for site publishers and search engines, but it’s not something that’s within reach of a lot of companies at the moment,” Potter says. “It’s very forward-thinking of Google. But the bottom line is responsive design saves Google resources, so they are trying to prioritize it.”

Google’s second suggestion for building a mobile site is to create pages that in the coding contain all the HTML required to build a desktop, tablet and smartphone version of the page. That way, all HTML is served from the same singular URL. This technique adds layers of complexity to site construction.

“The search engine and web server will detect what type of device you are using and based on that device a different HTML will be served from the same URL,” Potter says. “This is an option that gives a little more control over what kind of content is being served on devices, and it’s still easy for users to share and link because content resides on the same URL. However, this is something Google cautions against. Communication between the server and the search engine sometimes can go astray. I don’t recommend this option. It is out there for folks to explore, but it’s not ideal because of the ways web sites and servers work at this point.”

The third option is to build a separate mobile commerce site, which is what most retailers in m-commerce today have already done. Google says if a retailer chooses this route, it needs to use annotations in site code that indicate that a page is part of a mobile site and contains content that will be delivered to a mobile device.

“There are a lot of companies that have invested in mobile sites and they don’t want them to go away, so a way to mitigate that is to make sure that Google knows what it’s crawling is a mobile site,” Potter says. “That’s where this canonical annotation comes in, to indicate that this mobile content is the same as this desktop content. This needs to be performed on every page of the mobile site. What Google is doing is using the desktop ranking for the mobile ranking as well. But when you get the mobile search results page, the search engine will redirect you to the mobile page.”

So which avenue does Potter recommend retailers take? She says there is not an easy answer, and that she reviews web sites on a case-by-case basis.

“I’ve had two different cases recently,” she says. “One of them was a client that thought responsive design was great. We did an assessment of their site and it turns out it wasn’t a good fit for them because the way it was coded it would have taken a lot of effort and money to implement responsive web design. It was a design-heavy web site and there were a lot of constraints with that.”

The other client wanted a complete site redesign. “One of their requests was responsive design,” Potter says. “I would recommend to any client considering a thorough redesign that responsive design be one of their options.”

It all boils down to the state of a web site’s design, she adds.

“If you have a simple site it’s not that difficult to implement responsive design,” she says. “But if your site was coded 10 years ago and it hasn’t been updated in 10 years, it might be more difficult to implement responsive web design. It depends on the structure and design, there are a lot of variables, and we make this decision on a case-by-case basis.”