older a building is, the more chance there is that it’s listed. As a general rule, listing applies to all buildings built before 1700 and to most built between 1700 and 1840.
The vast majority of listed buildings are Grade II listed, but more important ones are Grade II* and the most important are Grade I. In Scotland, categories A, B and C are used instead of grades. The listing applies to the whole exterior and interior of the building, and sometimes attached structures and buildings as well.