So firstly; what is a flat URL structure?
A flat URL structure means that users (and search engine crawlers) can reach any page on your site in 4 clicks or less in a direct line from the initial click.
It was previously thought that Google favoured flat URL structures over deep and more complicated URL structures but they have now confirmed that this is not the case. This means that there is no need to artificially flatten your URL structure if it does not suit the site.
John Mueller from Google confirms that there’s no advantage to having an artificially flat URL structure compared to one that shows directory depth.
How many slashes there are in a URL has no bearing of how important a page is, or the position that Google will display the page in the search results.
The concept was discussed during the Google Search Central hangout on the 26th of March.
Someone asks Mueller what his thoughts are on short URLs versus URLs that show directory depth. He is of the opinion that it is advantageous to show users where they are in a site via the URL.
Mueller also agrees that the URL structure of a site can certainly be used however the site owner would like with no penalty in the rankings,
In Mueller own words:
“So if essentially the URL structure that you have on your site is something that you can use however you want. Google does not count the number of slashes in your URLs and say: “oh this is like five levels down therefore we will not show it as visibly in search.
It’s not necessary to have an artificially flat URL structure. That refers to a structure where it looks like every page is one click away from the home page, when in reality they may be several levels deep.
The fewer clicks it takes to get to a page from the home page is a signal to Google about how important the page is. But there’s no way to fabricate that signal with a flat URL structure.”
“You don’t have to have kind of an artificially flat directory structure. So from that point of view, if you have a directory structure that users can recognize and where you can tell that sometimes people are like even typing in the URL, or copy and pasting parts of a URL together, I think that’s perfectly fine. There’s no need to hide that kind of URL structure from users by doing URL rewriting or anything like that.”
Google treats URLs as identifiers of content, not as a way to understand site structure. That’s what Google’s web crawlers are for.
It’s completely up to the individual site owner whether they prefer a flat URL structure or one with depth. It will not help or harm a site by going either way.”
“For the most part we treat URLs as identifiers of content. We don’t try to understand the site structure based on the URL. So essentially setting up your URL however you want is our recommendation there. It’s definitely not the case that you need to artificially make it look different.”
See the full Q and A here:
So to summarise, it doesn’t matter to the search engines at all how you structure your site. As long as it is easily indexable, you should choose the structure that works for your site and more specifically favouring usability by the customers of your site.