Well, I'm no expert, but for the vast majority of people a detached storage space probably isn't even on their shopping list. And for most who put some priority

Well, I'm no expert, but for the vast majority of people a detached storage space probably isn't even on their shopping list. And for most who put some priority on storage space it will be fungible with an attached garage or full / partial basement, which are much more common. And it never comes down to these two homes are exactly equal but for the ADU. Logically, an ADU absolutely should be worth something but I don't see it reflected in my limited experience buying two properties with ADUs, one of them income-producing. Might be different for a rental ADU somewhere highly constrained on housing, but my rent-to-home-value ratio is comparable to what's being discussed here, too.