Perhaps read the patch notes next time?

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[2.0] To ensure that plots of land are not claimed and left unused, estates not accessed for extended periods of time will be automatically demolished.

Estates Subject to Demolishing

Free company housing that has not been accessed by at least one member of the company within forty-five days.
Private housing that has not been accessed by the owner within forty-five days.
Purchased plots of land upon which an estate has not been built within forty-five days.
* The count for days of inactivity will begin on the release of patch 3.1