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My prospect's domain ends in ".on.ca" (Ontario, Canada).

The structure of their site is companyname.on.ca (main page) and all other pages are sub-folders (companyname.on.ca/page-name-1

All pages are no more than two levels deep.

I'm wondering if anyone knows if the provincial sub-domain (.on.ca) presents an SEO challenge?

You're correct.  As a Canadian, I wouldn't be concerned with a county level domain.  For example: thismotel.ca but a .com would be better as they also attract customers from outside of Canada.

But they have a provincial second level domain.  .on.ca.  (thismotel.on.ca), no site can be found under just thismotel.ca and a search for WHOIS on CIRA results in this message "The domain name requested has usage restrictions applied to it."

Based on my research they've likely had this domain since before 2010 as now most provincial level domains are reserved for government institutions.

I'm thinking they will likely need a new domain for user experience alone, but I don't know if the fact they have a provincial second level domain has hindered them in the past with their SERP rank.

Thank you.