After contacting with my insurance agent to include this one in my existing policy which has few more single family properties and a blanket liability insurance, the insurance carrier (Indiana farmers mutual insurance) would need the last upgraded time or age for plumbing/roof/wiring to include it with other property insurance or to issue a separate policy.

For rule of thumb, through our in house program we developed with a carrier, replacement cost insurance is about .40 per sq ft.

Any recommendations on carriers who write aging commercial properties in WA state? 

I pay $150/month for a four unit in my area (that was the best price from a reputable carrier). 

I just got dropped by my insurance carrier (AAA). 

@Julia Amelyan there are many WI based insurance carriers you will want to try. 

Abdul King all the stuff that Kenneth Mooney said are great but let me tell you that just talking to the people even on the streets can bring you deals off market, here is why I'm a letter carrier (mailman) I got in a conversation with a guy in my route and result that he has two house that he's going to sell and he offered to me before they go on the market, so just talk to the people and try some of the above.

Normally I would recommend the LLC for a rental property, however, I would check to see if your current Insurance carrier will cover LLC's.  

I've been told that my personal insurance carrier would not add this 12-unit to an personal umbrella insurance, so apparently I need to look at commercial umbrella insurance?  

You should also get to know the postal carriers as they are among the first to know when a house goes vacant or someone moves.