The framers were at work again today prepping the second floor for so the lumber can be hoisted by the crane tomorrow.  The detail they were finishing was the second floor cantilever.  A cantilever generally protrudes out of the side of the building into the four foot setback area.  

The reason the side cantilever is only allowed on the second floor is to make sure there is room for fire or emergency access down the side yard.  Or at least I think this is the reason, I've never read an official version.  Regardless of the policy on cantilevers they require additional fire rating, and can be tricky to build.  They add cost in terms of framing time, fire rating material, engineering, and spray foaming.  The bottom of the cantilever needs to be spray foamed as that is the only way to really ensure it wont be a cold spot in the upper bedroom.  It also has to be strong enough to carry some weight of the roof.  A sagging cantilever would be a framing disaster we'd like to avoid.

It is debatable if the side cantilever is truly necessary. They are almost always featured on the inner city duplex because the houses are slightly too narrow at 21 ft wide to allow for two good sized second and third bedrooms. The cantilever seems to add just enough extra space to make the layout work.  Without the cantilever we have found that bedroom furniture can fit poorly.  

The detail we like to use is to notch the joists so that the top chord remains continuous where it passes over the wall.  I will show a drawing and the actual framing detail that was finished on site today.  It avoids a lot of messy dropped beams and ugly details that really hurt running ducting later on.

The photos below show the notched costs being placed over the double ply beam.  In photo 2 you can see the space just below the top chord of the joist where it was cut out.  Hangers and blocking are nailed on both sides of the beam, and from below you can see how the cantilever is hanging into space.  This design is extremely effective in this project because we have cantilevers above the front door and also out side side in the same corner area.  The third and fourth photo show this best.  I can't think of a better way to retain structural strength and have dual cantilevers.  Skilled framers make this look easy, but like most construction tasks it only looks easy until you have to try it yourself.