I went through the same dilemma last year when planning my ADU. I ultimately chose a detached unit, and it worked out well for me. Having a separate structure gave both my tenants and us a sense of privacy, which made renting it much easier. The upfront cost was slightly higher, but dealing with LA, Los Angeles, California
adu agency helped streamline the permitting process and offered clear guidance on setbacks and zoning rules. If you go attached, it can save space and money, but you may have shared utilities and less flexibility for renters. In my experience, detached units are worth it if you have the backyard space and want more independence for whoever moves in.