awesome.... Then you could take some 'electives' like Strength of Materials, Intro Materials Science (or whatever it's called) and so on... I just Googled for prerequisites and Stanford (just an example that popped first...) needs: Master of Science in Structural Engineering
Mechanics of Materials (CEE101A)
Geotechnical Engineering (CEE101C)
Structural Analysis (CEE180)
Design of Steel Structures (CEE181)
Design of Reinforced Concrete Structures (CEE182)
Programming Methodology (CS106A or CS106X)
So I'm guessing 4-5 courses minimum. Now, you could sweet-talk your BArch advisor to allow you to count some of these as electives (instead of, say, some arcane 'Studio 300 - abstract designs using drinking straws and toothpicks' to take a class like Concrete... Or they may be included in the BArch curriculum altogether... DD1's BS Arch has a couple of the above as dual-listed Arch and Civil Engineering...
As always, check with your specific school and advisor but I don't think they'll give you much grief. The above are standard fare civil engineering courses so they should be offered on a regular basis.