The Constitution's Fifth Amendment states: "nor shall private property be taken for public use, without just compensation."
Determining what exactly is "just compensation," however, isn't always so easy. The Martha's Vineyard Gazette has an account of the state of Massachusetts seizing a summer house that a family has owned since 1963. The state plans to construct a larger bridge on the land. The newspaper reports:
In an interview with the Gazette last month Mrs. Holloman described a tangled decade-long standoff between her family and the state over whether they would be allowed to stay in the house, and if not, also over what is fair market value for the property. The state has appraised the property at $267,000. An appraisal commissioned by Mrs. Holloman put the value at between $1.2 million and $1.5 million. The town assessed the property at $447,000 in 2012.
It's something for government entity, the town, to set one, higher, value on a property for the purpose of taxing it, while another entity, the state, sets a lower value on the same property for the purpose of compensating the owner for seizing it.