Hi Costas,
cardboad: this material is used in a wide range of things, for sample for box shoes. If you take a box shoes you have also four walls, you need cut windows, door and do the roof. Is basic but a good start. You have then the basic sape of a house. It´s time to do textures: brick, stucco, etc...
For more "professional" look you can go to artist suppliers shop and buy carboard of 2mm
best whises
CArlos