Prefab for ADU?

We’d like to build an ADU in our yard in Berkeley as economical as possible. We spoke to an architect and by her rough estimate, we could no where near approach paying for it.  But I’ve read the archives and it sounds like a few people have opted to do a prefab with a contractors help as a more affordable option.  Has anyone had experience doing this and if so could you give me some details on which prefab you used, which contractor, the budget, and how you navigated the permitting process in Berkeley?  I understand they are making it easier but after speaking with the architect the process still sounded pretty involved and complicated.