A more thorough explanation:

Definition: The Mansion-House Rule is a legal principle that states that if a property is located in two different counties, it will be assessed for property tax purposes in the county where the house is located.

For example, if a large estate is situated on a piece of land that straddles the border between two counties, the property tax assessment will be based on the value of the house and the land immediately surrounding it, and this assessment will be made by the county in which the house is located.

The Mansion-House Rule is designed to simplify the process of assessing property taxes for large estates that span multiple counties. By focusing on the value of the house itself, rather than the entire property, it is easier for tax assessors to determine a fair and accurate assessment.