Formal:
* Manor houses
* Landed estates
* Country houses
* Rural properties
* Country residences
* Chateaux (French)
* Haciendas (Spanish)
Informal:
* Country homes
* Farmsteads (if they are working farms)
* Ranches (in the US West)
* Estates (if the context is clear)
The best word to use depends on the specific context and desired tone.