what is the building line?
Depends where you are - Here in the UK it is a line marked on urban maps to aid planning decisions. In general you ar not allowed to build beyond the building line - often called the curtilage. http://en.wikipedia.org/wiki/Curtilage
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And in the U.S. it's called the setback.
It is the line around your property, and inside your boundaries, within which you may build. It will be some distance from the edges decided by zoning and planning laws.