If they know you, have spoken to you many times, and you have always been friendly with them, just explain your points, that there is no legal reason to deny your plans, and ask them what you can do to help get it approved.

This is a big legal issue that may weigh in favor of the investor. 

This doesn't replace the need for a MSA that will capture additional clauses needed on a Contractor-SubContractor relationship, this is just a project specific form you would use in addition to the MSA (prepared by your Attorney):This template is for informational purposes only and does not constitute legal advice.

Additionally, involve a real estate attorney to ensure the transaction aligns with legal requirements and protects both parties’ interests.

Perfectly legal, no hassles.

If you are a legal property manager in your state the owner should have the unfettered ability to allow you to act as his/her agent to enforce the lease, whatever provisions they may contain plus whatever your state laws allow.

This provision, through the Affordable Homes Act, make ADUs legal BY RIGHT in any single family zoned property in the state.

You could see an attorney or even better talk about it with your financial planner many have legal tech now that makes it essentially frictionless to get your estate work done and keep it updated.  

Because I'm not an accountant or versed in tax code and law wouldn't I want a CPA to ensure everything is legally tight and as advantageous as it can be? 

Make sure to get legal advice on what you can and can’t do for bill back and what kind of notice period is required in your state.