I was involved with a project where the fire department had a minimum 10' clear distance around the outside of buildings. This was so a ladder would have enough 'base' distance to lean up against the side of the building. But I belive that is more an International Fire Code rather than IBC.
There might be separation requirements based on fire rating of the wall as well.
On alot of the commercial design I see from architects they just make one large building and partitian it off inside.
Alot of times there are just lot lines separating the buildings and they come with thier own setback requirements from the lot line.
Do you have some more information you could provide, how close are you're two buildings, what type, stories, etc?