How much work do you actually need to do if you want to pitch your story to a publisher? I'd imagine a finished plot is advisable, but there's a lot of work beyond that. I'm asking because I have a lot of ideas that I want to explore, and what is reable to me, because I already have the images in my head, isn't necessarily readable to somebody else - at least on the same level of enjoyment that I experience from it.
I had a publisher talk to me over a contract, I had sent the detailed outline on one book and rougher outlines for the rest of the series. they then approached me because of that
I think it depends on the publisher! They will probably stipulate their expected level of completion on their query guidelines page.
By my understanding, a work of fiction needs to be complete, but non-fiction can be a proposal and sample chapters. The only exception to this would be a celeb or a proven author, as a publisher needs proof that the author has what it takes to complete a polished manuscript.
Unless you are an established and proven author, 100% of it. They don't want to talk to you about ideas, they want to see what you can produce. It's a bit different for non-fiction, but in genre fiction, you need to be able to hand over the whole book, complete and fully polished, before they want to hear from you at all.
Most publishers won't even look at your work without an agent. There are a few that will and have open submissions, but most do not. You need to present the best version of your manuscript before anyone is going to be interested. If it's an unedited pile of crap, you'll just get rejected. They may, and probably will insist on sending their own editor through your manuscript to fix the things they want fixed, but that doesn't mean you don't have to try.