I hope you do not mind the long answer, but it is a question that requires one.
A nonconforming use is a use of land or building that is not permitted by the use restrictions of the zoning code. A factory or junkyard in a residential neighborhood is an example of a nonconforming use. In contrast, a nonconforming structure is building that is permitted by the use restrictions of the zoning code, but fails to meet a site development requirement (setback, height, lot size, etc) in the zoning code. An example of a nonconforming structure is a house that was built according to the setback requirement in place at the time of construction (e.g., 75 feet), but does not meet the new setback requirement (e.g., 100 feet). While the Wisconsin statutes authorize the regulation of nonconforming uses, no such authority exists for nonconforming structures. (See. Wis. Stat. Sec. 59.69(10), 60.61(5) and 62.23(7)(h)) However, the nonconforming use regulations have been improperly applied to nonconforming structures over the years due to an 1997 Attorney General's opinion that failed to distinguish between the two.
In addition to the underlying premise that nonconforming uses must eventually be eliminated or brought back into conformity, the nonconforming use regulations prohibit the alteration, addition to, or repair of a structure that is being used for an illegal trade or practice in excess of 50 percent of its assessed value. (See. Wis. Stat. Sec. 59.69(10), 60.61(5); but see also 62.23(7)(h) which does not include the illegal use requirements.) The "illegal trade or practice" language is critical limitation to this provision, commonly referred to as "the 50 percent rule", but is almost always overlooked by regulators in the case of most structures. In fact, the statutes do not authorize placing limitations on structures where the structures are being used in a manner that is entirely consistent with the zoning code (i.e., houses in areas zoned for residential uses).
While the 50 percent rule may be reasonable for uses of property that are inconsistent with the uses of land in the surrounding area, this principle is entirely unreasonable when applied to someone's home or business simply because the development regulations have changed since the structure was built.
From a practical standpoint, the 50 percent rule prohibits homeowners from making routine repairs such as updating their kitchen, replacing the roof, re-siding the house, and other common improvements and repairs if the accumulative value of these and other repairs exceeds 50 percent of the home's assessed value. Although a home may have been built in accordance with the development regulations at the time, the 50 percent rule, as well as the entire non-conforming use doctrine, penalizes that homeowner simply because the development regulations have since changed.
Courtesy of WRA... more