Most people these days live in a building that was built in the last 50 years. In some countries the building age is older, some it might be only from the last few years, but how old would most buildings have been in the past?
Obviously that cannot be known, but was it common for families to live in the same house for generations? If I was wandering around 11th Century Britain for example, would people I know be living in Roman era houses? Just as some people today live in 15th C Houses?
You wouldn't find Roman houses still in use in mediaeval Britain, but you would find Roman fortifications such as town walls (eg the walls of London, 3rd century AD) still maintained and in use and at least two Roman fortresses (Pevensey and Portchester, both 3rd century AD) were in use as major castles. In the 15th century you would see a large number of buildings dating from or incorporating major elements of the 12 to 14th centuries inhabited. These included manor houses, castles and other aristocratic sites, urban dwellings, inns and institutional buildings such as hospitals and monasteries. Buildings were modernised and reconstructed to match changing fashions and rising living standards (especially from the late 14th century, which marked the beginning of the great rebuilding that extended into the 17th century) just as they are today.
In 11th Century Britain I don't think it'd be too common to find buildings as old as Roman. People generally lived in small towns and cities, such as York and London, or on small farms. I'm not saying there wouldn't be ANY Roman buildings, as there would ofcourse still be structures standing from when the Romans ruled, but I believe most of the buildings would have been built recently, as the average house probably wouldn't last very long.