AREA SEPARATION WALL Area separation walls are a form of fire wall which effectively divides a building into two or more separate buildings. They have many particular requirements which are best left to your architect to decipher, but there are some specific uses of area separation (a.s.) walls in live/work in Oakland. As noted elsewhere " the 10% rule"if one is converting a building to live/work and the designated residential area does not exceed 10% of the total building area and the number of units does not exceed 5. But what if you have a building you want to convert to 10, 15, or 20 units? By using area separation walls, you may be able to break it down into buildings of five units or less and therefore comply with the 10% rule, thus avoiding the requirements of a Change of Use, including seismic retrofit, closing up property line windows, and generally treating the building as if it were new.