First of all, just about any building can be expanded horizontally without any structural accommodations made in the first building. Very few buildings are built with that actually in mind, even though it can be done. However, you usually don't see rebar extending outward from a facade to eventually handle the expansion. As an architect, extending the rebar outward is an open invitation to structural deterioration and a huge source of leaks. There have been a few cases here in Fort Worth when that was clear intent, but no structural attempts were made on the buildings to accommodate the expansion. I remember seeing in old Star-Telegram articles when a historic skyscraper was built, the developer also bought the adjacent building that would share a property line with the new building. The old First National Bank (Bob R. Simpson Bldg./Baker Bldg.) was an example of this. There was one hybrid example here in Fort Worth of where three horizontal additions to Stripling's Department Store were built with all three designed to be expanded vertically from 3 stories to 7. Two of the three buildings were expanded to 7 with the third, not receiving the vertical expansion. On the upper floors, the steel beam and rebar were exposed. Here's a link to a historic photograph below from the UTA Digital Galleries Website.
https://library.uta....99/20089067.jpg
Finally, when they demolished the Landmark Tower, you could see the structure of the lower part of the building. There was a large bridge beam on the north side of the building carrying the loads of the upper portion of the building. Where this beam was located was really not needed unless there were plans to expand the banking hall to the north at some point in time. One of the features of these grand banking lobbies were an almost column free space. This bridge beam aligns with the large spans inside the building. Like the First National across the street, Continental National did purchase the building next door at some point before the skyscraper was built. I don't know when that purchase occurred, but Continental National did occupy the old Fort Worth Board of Trade building on this site prior to 1921. In that year, the bank moved to the National Bank of Commerce and relocated into the W.T. Waggoner Building. I do know that at some point in time afterwards, Continental National was the owner of the Board of Trade Building. Please excuse my tangent, but here is one of my photographs during the demolition phase.