My house is a two level, daylight ranch built in 1951 on a slope with the top floor exiting at ground level in the back and the bottom floor exiting at ground level in the front. Subsequent to initial construction, a 10' x 16' cantilevered (overhanging) extension of the livingroom on the top floor was added in the front of the house. This extension is supported by 3 posts under the exterior wall of the cantilevered extension. The floor joists of the cantilevered extension sit on a 2x6 sill fastened to the side of the house and are toe-nailed to the header joist from the main house.

This is in an earthquake prone area and the existing structure does not meet current code. I plan to make some modifications to the support posts and have been told that if I make changes I will need to "tie" the floor joists of the overhanging extension into the floor joist system of the rest of the house. Fortunately, the joists of the extension are parallel to the joists of the rest of the house (and, they run perpendicular to the wall against which the extension is attached).

Question: If I open up the header joist between the extension joists and the main house joists, can I "sister" a reinforcement joist to each pair of extension and main joists? And, if that is possible, how long does the "sister" reinforcement need to be?" If I use a 10' sister on each pair of joists, I would have 5 feet on each joist assuming the reinforcement was equally spaced between the two of them. I do NOT have much more room than 10 feet either from the inside or from beneath the overhang on the outside to use anything longer than 10 feet.

Any guidance and/or suggestions would be greatly appreciated. Thanks

You need to check with local building authorities, especially since you are in earthquake zone. You do not have a cantilevered addition. You have an addition that one side rest on the existing main wall and the other on the three post. Toe nailing certianly won't do, even in a non earthquake zone. Wood needs to rest on wood or use joist hangers. Check with authorities. May be that metal plates could be used to tie joist together. Could be that just joist hangers would do. May have to bolt plates together.

Thanks, hkstroud. That is helpful. By knowing some of the possible options, I can better discuss alternative solutions with the local building authorities.
I realize that it is NOT technically a cantilevered addition; I only describe it as such to help convey the image of it hanging out 8 feet off the ground from the main house (but supported by 3 posts). To clarify, the floor joists of this addition DO sit on wood, but it is only a 2x6 sill fastened to a support beam on the main house. The ends of the joists are then toe nailed into the header joist which is shared by the joists in the main section of the house. Again, thanks.

You have a rim joist and the floor joists should be attached with metal joist hanger. You might still be able to add hangers if you can get under that space.