Backyard ADU: how to create its independent entrance?

[Zi Wang]

Hello, I'm new here and also new to real estate investing. I'm looking at a property in inner city with a pretty big backyard (an around 1800 sq ft backyard; small for suburbs but already quite large for cities). It's perfect for building an ADU but because the property is an attached single family and its backyard is surrounded by other properties' backyards, it almost seems impossible to create an independent entrance for the ADU. I was thinking about buying others' backyard but it might be pretty cost ineffective. Another solution would be to build a "tunnel" through the main house, but that will probably also be pretty expensive.

I'm wondering if anyone else has run into similar situations before, and what would be your solution to this scenario? Thanks!