There is no such rule. Leaving one wall standing will not grandfather a non-conforming structure. Once the cost of rebuilding exceeds 50% of the equalized assessed value of the existing structure, it is considered a new structure that must meet all current ordinance requirements. Questionable estimates of building cost will be calculated by the zoning office using nationally recognized standards.
Am I grandfathered?
"Grandfathered" is another term for existing non-conforming structures or uses. Please see the following answer for non-conforming issues.
Nonconforming – what is it?
There are both non-conforming uses and non-conforming structures that are regulated by County zoning. These are often referred to as "grand-fathered."
Non-conforming Uses involve the existing use of a property that is in conflict with current zoning. It may be a house located in a commercial zoning district, a duplex in a single-family zoning district, or a house in a residential district that has had an insurance office in it for the last thirty years.
The lawful non-conforming use of a structure may continue even though it does not conform with the provisions of the zoning ordinance until the use is discontinued or terminated for a period of twelve (12) months. After 12 months, the new use of the property must be in compliance with current allowed zoning uses.
Lawful non-conforming structures begin at the time they were made non-conforming by the terms of the current or previous zoning ordinance. These structures may be located too close to a lot line or another structure; they may be located in a floodplain without meeting elevation requirements; or they may be located too close to navigable waters.
Total lifetime structural additions or alterations shall not exceed fifty (50) percent of either the current equalized value or square footage unless the structure is permanently changed to conform to the ordinance. In the shoreland jurisdiction, only the value method may be used.
Please connect to our link on nonconforming structures for detailed information on how these restrictions are applied.