I had no trouble filling my rooms and the house appreciated for $10K over purchase price, the tenants are paying utilities. 

I have been telling my sons to utilize that!

Eliminate debt, establish a budget, and save.

The thinking on this is that although the buyer utilizes what appears to be a purchase contract, it is actually being used as a purchase OPTION, and as such an option contract which clarifies that the purchaser is actually obtaining an option to buy should be used.   

Monthly Expenses: I’m accounting for rent, utilities, and cleaning supplies, but are there other hidden costs I should consider?

You can utilize hard money for a BRRR or flip to get into the game. 

If the property is sub-metered/ if you intend to have the tenants pay utilities make sure to mention that to them. 

Are you utilizing REPS?

It now cash flows beautifully, which I continue to utilize for other my other properties. 

They need to do all their due diligence including understanding the value that will be added by the ADU, the finance options available, the consequences of adding the ADU (rent control, property taxes, rental limitations, etc), etc.Good luck As with all things, it depends on the investor and the equity.If they are able to leverage existing equity to buy it under a HEL or HELOC, then they are adding equity with no out of pocket expenses, and adding immediate and longterm cashflow to existing stock.We like ADU's because there isn't more land being created in our city that is landlocked...but there is more density available to those with large lot under-utilized sfh.In my market, it typically costs $2 to add less than $1 of value on single build ADUs.