With a 600' separation it sounds like you have a reasonable sized piece of property. Think into the future for some number of years. What is your exit strategy for your business? Or for your building if your business becomes sufficiently successful it outgrows your location? If your building can be split from your residence it may make selling your business easier, or allow you to sell the building if you sell your business to someone who doesn't want the building.
Just because your area isn't zoned now doesn't mean it will never be zoned.
I agree with all those who say build the building personally, and then lease the building to your business. This allows you an avenue to receive income from the business without paying SS, WC, UC, or other taxes on that income. If you have all your available cash in the business, the business can even loan you your money so it doesn't have to be taken as income and rinsed through the tax man first.