@Levi PolterShopping around on Biggerpockets is the first step.  

Market rent is about $2,200 right now, and could go as high as $3,000 if I added some sparkle (or an additional bedroom).My wife and I wants to buy it from him, but we're trying to figure out the best way to do it to reduce taxes (property / assessment, income, excise) and puts us all in a better financial situation (short-term and long-term).If they were to sell it on the open market for $300k, they'd incur 10% selling costs (closing costs, realtor commissions, excise tax) of $30k and get $270k in cash before taxes. 

@Levi T.

For owner occupied, they are capped at 1% of assessed value (+ any special levies that voters approve). 

He had no clue WA charges excise taxes each time title transfers.  3.4% on the buy/sell, whether you own for a minute or a decade.  

@Levi T. - That is a very sad and weird one.  

If you have none of these things, then as @Levi F.

If you're trying to add more rooms so that they are legal additions, you can often sell the house for more and legally advertise it as a bigger house, but keep in mind that MOST places will levy a higher property tax assessment as soon as you finish the addition.  

They can only raise their levy by a certain percentage.

Property tax in Portland jumped with all the levy’s and assessments that hit this year (and I don’t think that’ll stop in the future) while the city council wants to blame the homeless situation and housing crunch on landlords and have us be the only ones to foot the bill.