One Acre Buildable Rationale

As more and more property is developed in Chisago County, it is inevitable that marginal pieces of property, which may not be suitable for development, will be proposed for subdivision.

In July of 1997 Chisago County began requiring one acre of buildable area for all newly created parcels of land. This requirement applies whether the parcel was created by metes and bounds or formally platted.   The County’s definition of “buildable area”is found in Section 3 of the Chisago County Zoning Ordinance, and reads asfollows:  An area of land excluding slopes greater than eighteen percent (18%), surface waters, wetlands or flood plains and where the depth to mottled soils is at least one (1) foot.   Chisago County Zoning Ordinance Section 5.15 (Dimensional Standards) stipulates aminimum of one acre of buildable land be established for any new tract of land being created, regardless of size.  

This requirement was adopted as a minimum “suitability for development” standard and has nothing to do with the actual septic system design or installation.  Chisago County’s philosophical stance toward development of its open space is generally liberal, but does include this proviso:  If it is impossible to identify at least one acre of land on a proposed lot that is free of extreme slope, surface water or wetland features, extremely high watertable, or fragile or organic soils, then the property is considered to be unsuitable for development in that size or configuration, and it is deemed “unbuildable.”   In other words, the County’s minimum lot size in the Agricultural zone is five acres.  If it cannot be proven that at least 20% of such a tract of land is buildable, then the County prohibits development on that property, including new construction of a single family dwelling.

It should be noted that the one acre of area does not need to be contiguous, in other words, 10,000 square feet of area could be found in one location on the property, 10,000 in another, and the remainder somewhere else.  

When a septic system is installed it must be constructed on original soils with at least a 36 inch separation from the bottom of the trench to any evidence of mottling. The depth at which the mottling is found will determine what type of septic system will be required. If mottling is foundwithin the first 12 inches of soil no septic system, including a mound, will be allowed.

On the average septic system installation requires that approximately 5,000 square feet of area be tested (soil borings and percolation rates).  The one acre of area was chosen as it would be a way to determine whether or not the new parcel would be able to support a new house, outbuildings, septic treatment, and back up septic sites for the future.

Mottling can appear as splotchy patches of red, brown or gray in the soil and can show that at some point in time the water table rises to that point and remains long enough to cause ‘iron staining’ or anaerobic conditions.  

The mere location of a property cannot tell whether or not the area will be buildable. Property located by a river or low area can sometimes perc fine dependant on the soils. For example, many of the areas along the St. Croix river in Chisago County are actually comprised mostly of gravel thereby allowing not only septic systems, but in ground trenches whereone might think a mound system would be required. On the other hand, for whatever geological reason, perched water table, etc., some sites including those on a hill may not pass soils tests.


When it happens that a person brings a soils test (perc test) to the Environmental Services counter staff cannot tell them immediately whether or not it ‘passes’. Staff can examine the findings and discuss the type of system that should go in provided the test is correct.  Until the soil borings are actually field verified by zoning staff, and they concur with the tester’s findings a permit won’t be issued, or a plat approved. 

Unfortunately, soils identification is somewhat subjective.  On occasion a soil tester will not call mottling within the first twelve inches, but during groundtruthing several members of the Environmental Services staff will concur that it exists. Since staff has undergone extensive soils training, has had years of field experience, and is unbiased (as not being hired to conduct the soils test),their decision is considered to be final. This is the best way to be consistent in a sometimes difficult situation.


For questions or additional information, contact 651-213-8379.