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.

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.

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

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

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.

In the end our actual budget has been ahead of our estimated which has worked out in our favor.The biggest variable is going to be placement costs.

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

Get plans drawn and get estimates from subs to get a total cost price for property.