Is it possible to find the original building permit plus floor plans for houses in the city? Where would I go? City Hall? Library?

Floor plans are rare. City Hall, Room 1 has building permits on microfilm; they go back usually into the 1870s at the earliest, but actual addresses only show up starting in the 1890's. Until then it is just descriptions like "south side of Arsenal west of Kingshighway) so they can be a little difficult. Also, if you write down the permit number and date, you can usually find a notice of the construction in the Daily Record which will tell you some more information about heating system, roof type etc. and usually is more accurate in terms of architect and contractor than a permit. Central Library has Daily Record on Microfilm, but it is in storage. Landmarks Association also has Daily Record on microfilm and are very friendly about pulling something small like that for people.