Stuck on choosing the broadest niche for authority site.

I'm wanting to start an authority site which I can promote affiliate products on and get traffic from Facebook page, Pinterest, Twitter, etc, and then eventually traffic from the search engines. I've read the courses 'RankXL' and 'Affilorama' and they both say that the best way to go about it for the long term is to build out an authority site with lots of different sub-niches/sub-categories. This makes sense to me and that's the way I'd like to plan my site, but I get confused when I start thinking about how broad I should go with the niche.

For instance, if I want to do a site on gardening, that's a pretty broad topic and has many different sub-niches that I could write about for years, but should I go even broader than gardening? Where does it end? And if I do go too broad, and I have social media pages for the site, wouldn't people find it strange if they're following a site about gardening as well as something completely different?

So basically, when planning out an authority site, how broad should you go with the niche?


Gave IM a little bit of an attempt a few years ago, but never really took it seriously. Now I've got more time and money, I can give it a proper effort.