Hi Joshua!
This is only my opinion, and full disclosure, I am a vacant land wholesaler...
That is a good plan, again, just my opinion. I would ask the smaller builders around your area if they would "fee build" your home/ADU. I have heard from a couple of my builders that cost plus $25,000 is about where they will start. I say start because they obviously make more building for themselves. But if you have a relationship with a local builder, you might have an "in" with them.
Look at comps for what new construction homes are selling in the area you'd like to build in. My clients are looking to buy the dirt anywhere from 25%-30% of what a finished, new construction home would sell for on that dirt. That is for a finished lot, stubbed with the utilities, water, sewer, power, etc... So a new home selling for $400k, expect to buy the dirt under that house for about $100k...
Utilities are a real challenge and what kills a lot of the deals that I write up for my clients. Especially in the parts of King County where there is no sewer, or its good distance away. Be careful about some jurisdictions...if sewer is a certain distance away, some local governing bodies will require you to connect to sewer. At about $200/ft, if sewer needs to be brought in, it is not cheap. And that is not including the hook-up fee, or if you have to add a pump. Some jurisdictions even require you to underground the power, like the City of Burien. If the power pole is across the street, you might have to cut the street or drill under the street to bring power to your lot...and that can be really expensive.
I have heard some people have no problem getting permits to build for their vacant land, and others have had great difficulty. You may want to invest in a land use consultant. They usually have a contact at DPD that can help you through the process with less hassle. But you can also tackle that. Just know that it takes a good amount of patience.
Three words: Environmentally Critical Area. Look out for this, its a pain to deal with, but can be dealt with. Usually includes extra permits and feasibility study time and buffer zones that you cannot build within. It can also mean wetlands, and the buffer zones for those...
The last thing I'll mention...Trees. In City of Seattle there is a Significant Tree ordinance... certain trees are not to be cut down or removed. You also need a City of Seattle arborist (of which there are only 5 the last time we had to look into this) to come take a look at the tree to see if its a rare type that must stay. Sometimes there are pricey permits to bring them down. This can affect the footprint of the house in a negative way. Just something to look out for.
There is a ton more info that I would share with you, just from my small amount of experience, but those are some of the bigger points that I would keep in mind. I'd be happy to share more if you are interested.
Cheers!
Brian Truman
