Say I have a 2 y.o, website in the money making field, indexed, some ranks, visits, PR etc. I have the intention to build a subdirectory (not a subdomain) covering a slighty different niche. My first idea is to go ahead with this since I have a lot of opportunities to build link popularity and PR for the root level domain, and with some time I think this will be beneficial for the SE rankings of the subdirectory. In this subdirectory I'm planning to target niche keywords (not competitive ones). I'm wrong with this ? Or it is better to build a new website? Thanks
If it's a continuation of the original site then sure use a sub-directory, but if it's going to be totally different as in the style of writing, layout, etc... etc... then I suggest you buy a domain and start fresh. The reason I'm saying this is not for SEO purposes but your site will be more valued on the internet if it runs on it's own domain. What happens if you ever want to sell the site? Hmmm... you can't can you
Sub-directory is fine as Danny mentions, but you need to consider the keyword proximity. If the niche can be broken down into subcategories, creating a sub-domain would be better. ie. keyword-subdomain.tld-domain.com I've done this on a few of my sites, creating about 10 subdomains and broken down into categories within the health sector, then linking up related articles to other subdomains on my main domain, works wonders.