I am working for a non-profit Canadian organization. We requested a quote for renovation from several contractors. Do we need to notice all the contractors who are not selected?
It's always a matter of good courtesy to do so. Nothing worse from their perspective than waiting for some response and perhaps turning down other work in expectation of receiving work only to hear it was decided a month ago and that they didn't get it.
In my opinion, yes, it's the professional thing to do. You may have to work with some of those contractors on other projects and a courtesy note saying that you thank them for their effort, but weren't successful, to be sent out as soon as possible after you've selected your contractor buys a lot of future consideration. The construction business is too adversarial sometimes, and if you let the other contractors know that you appreciated their effort, it at least acknowledges that you've given them a fair shot. Now if Clients could also treat us consulting engineers that way as well, maybe we'd see some better consideration from them too.
If I were the contractor that was not selected, I would ask for candid feedback on why not so that I might be more competitive in the future. Was it labor rates, overhead, quoted hours, material, timeliness, or maybe proposal quality? I'm not sure how large of a job you're talking about or the likelihood of future projects, but you might help someone out if you've got the time or inclination to explain your selection criteria. You might build a relationship for future work.