Raising rent in Seattle

[Laura L.]

Have a tenant on 15 month least that is expiring in Sept. They likely want to stay. Makes sense, as they are paying $1,575 in a nice area of Seattle, on the rapid bus line, secured bldg, W/D/ GD, DW, Stainless/Granite, WSG paid, 10 minutes from downtown, in a fully updated unit with storage locker and two off street parking spaces. Rents went up in the area IMMEDIATELY after they signed a 15 month lease. We are now 5.5 months out from the lease expiring, and I need to raise the rent. 

Rents in the area for similar units are going for 1,750 to 1,850 now, and this is the off season. That's $275 max difference. 

This seems like a drastic amount to raise rent. But, based on the area, I know there will be other tenants who will pay this, and I do need to let the current tenants know rent is being raised. 

Question: How do I go about letting them know this, in a legal and appropriate way? I already emailed them in February and said that we needed to talk about changes that were coming in September. We have not had the conversation yet, but need to this month. 

Sorry for the stupid question, but this is the first time I have had to raise rents like this.