It would almost certainly depend on the country, but if the structure has some kind of copyright, miniature replicas would likely not be legal without permission.
In some countries, building facades and similar can be copyrighted, which is mostly discussed in the context of photography. Some countries have an exception called "freedom of panorama", which gives some fair-use rights for photographs of things that can be photographed from a public street. How broad such permission is varies: in some jurisdictions it's fairly broad permission, while in other countries it may only apply to photographs of a general street scene that contains the building as one element, but may not apply to detailed architectural photography focused on a particular building.
But that's only for photography, at least so far, not for 3d reproductions. To take an existing area where the distinction between the two comes up, in some jurisdictions you can take (and sell) photographs of sculptures that are installed in public squares under freedom-of-panorama, but still cannot sell reproductions of the sculpture, whether full-size or miniature, if it's new enough to be copyrighted.
The above doesn't typically to things old enough to be out of copyright, like Big Ben, or Rodin sculptures, though I wouldn't be surprised if some jurisdictions have special-case laws about use of their famous landmarks.
