WA state real estate law, land use and zoning issues, open spaces zoning, procedure for changing zoning status

Are you located in unincorporated Snohomish County? If so, you can view Title 30, the Unified Development Code here: http://www1.co.snohomish.wa.us/County_Services/county_code.htm.

Yes. Open space zoning is not your average 'you can build a house' (or for that matter anything else) area. The zoning rules are going to be somewhat different for each municipalitiy/ government entitiy. For example, the City of Snohomish has an OS (Open space) zone which is mainly parks and ballfields, City of Arlington does not have an OS zone, Snohomish County has agriculture, forest and open space zones. You are going to need to check the rules which apply to your property. But, in general you are probably going to need to change the zoning if you want to build a residential dwelling unit. Just contact the planning department which has control of your property and ask for their help in gathering information. At some point you are probably going to need the assistance of an attorney or a planner or land surveyor to help you with your project.

This answer is for informational purposes only, does not create an attorney-client relationship, and please note that no specific research has been performed and the law in your jurisdiction may vary. I recommend that you hire an attorney.
If the zoning has removed all value from your land, or violated your reasonable economic expectations, you may have a cause of action for "inverse condemnation." Are there any uses under the current zoning which would accrue an economic benefit to you? Are you still permitted to farm the land? Are any aspects of your use of the land grandfathered -- that is, were they already being done before the zoning changed, in which case they may constitute a prior, nonconforming use? Did the prior zone permit farmhouses? These are some of the questions you should ask your lawyer. I see one answer already provided you with a zoning code. If that is the code for your location, you may be able to find some answers there; for example, whether or not the zoning permits one house, and if so how many acres are needed.