The people to talk to at the county are at Community Development - 360-379-4450. As long as everything (houses, water source if necessary, septic if necessary) fits on the property with the necessary setbacks, you can build a primary and a secondary house (ADU). Being able to sell them individually at a later time is another matter. They have to be on separate tax parcels for that to be possible. If there are already two parcels, you are fine. Depending on where your property is located, you may or may not be able to subdivide it. Much of the county can not be newly subdivied into parcels smaller than five acres. If your property is in that category you would have to have 10 acres to be able to sudivide it. Again, give the community development people a call. They can answer a lot of your questions over the phone.