staying in Kiso Valley

Is it essential to pre-book accommodation if visiting Kiso Valley, especially Tsumago or Magome? Is it possible to just arrive and find somewhere to stay? (will be visiting mid-September)

Most of the accommodations are small and family run. Many of them prefer to have advance notification because meals are included and might require advance preparations. As a result, they might not be able to accommodate you even if they have vacancies.

Of course, there is also the risk that they are booked out.

In other words: Advance reservations are highly recommended. At least half a day in advance.

I would pre book. I don't think it would be easy to just arrive and locate accomodation. In Tsumago for instance, it was hard to even recognise which places were ryokans. We had to take a taxi to our ryokan and the taxi driver pointed out the place. Without that I don't think we would have ever found it.