Either the owner takes risk by converting the 5th unit to an accessory use or you take the risk by, for example, increasing your non-refundable security deposit so s/he can convert it prior to close w/o financial risk in case you don't close.At the end of the day, these are the costs of putting yourself in situations where you're not following all rules.  

At this point, we can stage a typical 3 bedroom, 2 bath, 2000 sq foot house (including all furniture, pictures and accessories) for about $3000 or less.

Depending on how many units per the parcel, it can be viewed as a SFR with an accessory unit, it can also be viewed as a duplex, tri-plex, or 4 plex? 

Does Anyone know if it is possible to have an accessory dwelling unit in this type of zoning?

You could build an accessory dwelling unit (ADU).

So my question is, would it seem economically smart to buy a vacant lot, use a home builder to build a house and possibly put and ADU (accessory dwelling unit)on the property as well to cash flow?

Big box will lull you in with essentials and get you with accessories.

Here is the Urban Growth Area Map.http://www.co.thurston.wa.us/planning/comp-plan/docs/M-14%20ugas%20for%20land%20use%20chapter%20gina.pdf Here is a quote from the Thurston Co. website:http://www.co.thurston.wa.us/permitting/faq/faq-aboutproperty.htm  Can I build an accessory dwelling unit or an accessory living quarters on my property?

I will bet they'll come back with you can add an accessory dwelling unit which in my opinion isn't worth it.

If we sink more money into the rehab our cash on cash return will likely dip below the coveted 20% mark, unless we end with increased revenue as a result of the additional kitchen rehab.