During the second half of the twentieth century, however, those glories faded and Penllergare began its long slide into dereliction. The mansion was destroyed and replaced by a ‘civic centre’. Development and vandalism added to the effects of neglect. The woodland gardens were "top-sliced" by the M4 motorway. Modern houses abut on the walled gardens and spill into the park. The promised country park in the 1990s failed to materialise, and the derelict house was demolished for safety reasons in the 1960s.