My primary house estimate value is around $450k, I still owe $60k for mortgage, and I have $45k in high yields saving now. 

The Triple Net Component of the Lease is paid by each tenant on a monthly basis and is an estimate of what the total expenses will be on the property for the year.

For perspective, the house directly next to this house is estimated at over $900k fully updated with slightly higher square footage, but 1 less bedroom.

I'm wondering how folks estimate CapEx, maintenance/repairs, and turnover costs?

There are drawbacks however, and it certainly isn't cash as multiple draws, estimates, and appraisals are required throughout every step of the process. 

Never, Never use your sellers property tax expenses to estimate your future property tax, this is a big mistake done my many people in the industry.If you haven’t sell your Main home where you are living yet you might consider to keep it, because your new home would be triple the amount your are paying right now.

Having engineering plans and calculations can streamline the permitting process, as many local authorities require this documentation for ADU conversions.I can't give an invoice estimate as I am not well versed in how engineers price their work, but I would assume something around $5,000 for engineering services on a project of this size.

I estimate construction costs, create plan sheets, and project manage for my every day job.

Meetings, emails and calls followed where I tried in vain to nail down an estimate of cost.

The last property I sold the realtor listed it for 10% over the Zillow estimate and we were under contract in 2 weeks.