@Ramsin Jacob you should be able to do this no issue. 

Note this is the biggest issue, but other things to consider is no revenue until ADU is built and rented.  

I would start simply by issuing the tenant a written lease violation warning for denying access to evaluate any potential issue.

It turned out to do very well for us and the owner, as there were no maintenance issues except for one larger issue that had to deal with old pipes.

Hi Quinton, I'm currently building a software app to help easy manual and transparency issues with RUBs and offering smart device integration for submetered properties.

If I was your brother getting my REI portfolio started, is there any advice, strategies, links, literature, articles, or people to follow that you would recommend for someone extremely leery of financial risk?

I only have to jump in when there is a specific question or issue from a guest. 

IRS audits and penalties: Studies that are poorly conducted may be full of unsubstantiated assumptions and errors that can lead to expensive audits and large financial penalties from the IRS.Long-term issues: Dealing with IRS audits and having to correct inaccurate studies can be a drawn-out, stressful and expensive process.

It does not matter if probate was opened or any notices sent out or anything similar.I understand your reasoning, but it is a constitutional due process issue that you cannot take someone's property away from them without due process of law, which includes notice.

Lastly, being a good landlord will always bear fruit so it's best to be on top of every issue that arises.