Converting Bonus room to studio ADU

[Angela Zaitz]

Thinking about buying a small home that has a large 300sq ft bonus room and converting it into studio. I would need to have a kitchenette and small bath/shower added. I would live in studio and rent out main home. 
The bonus room is close to the kitchen so water would be close. But I wouldn’t want bathroom next to kitchen, that doesn’t seem like a good plan.

Is there anything a newbie may miss on a project like this? I will not be rehabbing myself, but hiring contractor and requiring them to get permits.

Any idea how much it would cost? And besides getting multiple estimates, any way  to get a ballpark estimate ahead for planning purposes? I don’t think I can get a contractor out soon enough to have info before needing to make offer  homes are going like hotcakes here and contractors are backed up and I don’t have a relationship with one  

This is my first project and investment property. I know I would need to factor in utilities into the rent. anything else I need to consider ? 
Thanks in advance for your feedback and sharing your expertise!