ADU Construction Costs

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I'm looking at a property with a detached garage (550 sq ft) built in the 50s and am researching the costs of

1. converting the garage into an ADU, or
2. adding an ADU on top of the garage

The property is in Tigard and has a SFR on it. Adding an ADU on top would allow me to keep the garage (which is valuable to most people), but I'm guessing is significantly more expensive than just converting the garage.

Does anyone here have experience building ADUs and have an idea of how much I could expect to pay? My research so far has shown a really wide range of costs.

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Portland's own Kol Peterson just wrote the book on this:  

https://accessorydwellingstrategies.com/backdoorre...

In fact, there is a book launch party this Tuesday. 

He also maintains a very informative website: http://accessorydwellings.org

If you attend the book launch (Google the book title on eventbrite), you'll be able to ask Kol about your specific situation.