I actually like them because the homeowners' association (HOA) usually covers most of the insurance, and there is very little maintenance required.

Timing will make a big difference leasing this SFH in Seattle— if possible, aim for your vacancy to hit the market ~May 15th to June 15th, maximizing time on market for a June / July lease start date— this will give you the largest tenant pool, lowest vacancy time & highest rent.

Most of the time, an ADU will cost 100k just for that part and then you have to actually build the cosmetic part for an additional 40-50k. 

If it is a high-end project, I would spend on an actual lIcensed designer.

Actually, seems like you AGREE with me

I actually have a few deals under 75% ARV right now.

Cleaning up our work area, I remembered what my old co-worker once told me on the construction site "There are 3 ways a job gets done: One way in our head, another way planned out, and how the job actually goes." 

It likely is out of date as to cost as this info is quite old (my guess is 5 years but it could be longer):- Rent Recovery Service: offers flat fee service starting at $20. 3 letters start at $30.

I personally would sell the house - it's negatively cash flowing ~100/month, if you were to add in repairs/maintenance, CapEx, vacancy, rising insurance/property tax costs I would think you're actually losing more than you estimate (luckily it's new so you are on the very low end of that right now).

You said in your post  "If the notice was delivered by mail then the notice period begins when the Tenant actually receives the notice."