Buildings are always being changed. This includes the addition of extensions, partial demolitions, structural alterations and changes of use. The structural engineer has to understand the way the building was originally built in order to design safe and sympathetic alterations and to check that the elements of the building will not be overloaded by the new work or use. Few building owners are able to supply the original construction plans. With traditional building types it is usually fairly simple to understand the structure but with more modern buildings this becomes more difficult and site investigations may be needed.