Stepped Building vs. Zero Lot Line Wall

[MKALLAY]

I am looking at a condition that involves a new building proposed on an urban site, with one wall being an existing party wall along one property line. The existing building on the adjacent property is two stories. The proposed new building will be 5 stories. The designer believes that the stepped building provisions of 706.6.1 will permit them to reduce the fire rating of the exterior wall on the shared lot line and provide window openings at the upper floors. I believe that notwithstanding the provisions of 706.6.1, the exterior wall of the new building above the party wall must also comply with Sections 602 and 705 based on fire separation distance (which, in this case, means no exterior openings permitted). Is this correct? Or does the designer have a legitimate argument?

Just for an academic argument - would this also apply if we were looking at two separated buildings on the same lot, i.e., does the imaginary lot line then extend above the lower separated building - which would then seem to make Section 706.6.1 moot?

(Note the code jurisdiction uses 2018 IBC)

 

[steveray]

Yes...the lot line extends up...

 

[classicT]

The lot line is actually a plane that extends vertically. That said, is it a real lot line? You mention party wall and imaginary lot line. If that is the case, then FSD may not be the right path to look at. The party wall would be a fire wall which creates separate buildings for limited purposes.

 

[Paul Sweet]

The reduced fire rating and windows won't be much good when the shorter building next door is torn down and a new building that is taller than 5 stories is built.

 

[MKALLAY]

The reduced fire rating and windows won't be much good when the shorter building next door is torn down and a new building that is taller than 5 stories is built.

Agree, which is why I think (and the designer in the case above agreed) the Fire Separation Distance requirements should govern in a true lot line condition; the stepped building provisions in 706.6.1 would seem to only make sense for a firewall condition where a structure on a single lot is divided into two buildings with single ownership.

That said, as an aside - at least one jurisdiction (New York City) DOES have provisions that permit zero lot line windows in limited conditions with special provisions - including that the property owner of such building agrees to close off such openings in the future if a structure on the adjacent property is enlarged. Just goes to show, all Codes are local!