Comprehensive and Detailed Explanation From Exact Extract:

According to the International Building Code (IBC) and reflected in the NCARB Project Planning & Design guidelines, when an existing building has reached the maximum allowable floor area, horizontal extensions (additions) can be made only if there is a proper separation between the existing structure and the new addition. This separation must prevent fire spread and protect structural integrity.

A fire wall is a continuous, fire-resistive barrier designed to prevent the spread of fire between adjacent buildings or different parts of a building. It must have structural stability under fire conditions and can allow the two portions of the building to be treated as separate structures for area calculations, effectively permitting expansion beyond the allowable area of a single building.

Fire-rated shutters (Option A) are movable devices used for compartmentation but are not intended for permanent separation of building areas.

Outside space separation of at least 10 ft (Option B) is generally insufficient for full area separation unless it meets specific fire-resistance rating and distance requirements that typically exceed 10 feet.

Water curtains (Option D) are active fire protection devices but are not substitutes for passive separations required by code to extend allowable building area.

Thus, the correct method to extend a building beyond its maximum allowable area is to separate it with a fire wall, as per NCARB and IBC provisions.