There is a lack of guidance dealing specifically with foundations for low-rise buildings extensions, particularly for housing. Local practice, often established through rule-of-thumb, can vary, while more formal procedures usually follow guidance for entire, new buildings. Owners and their builders may be required to adopt extension foundation depths that differ markedly from those of the existing building, which can lead to confusion, dispute and even damage at the junction between old and new structures. This Good Building Guide suggests procedures to avoid these difficulties. Options for dealing with problems such as shrinkable clay with trees, loose, compressible fill, or soft compressible soil are suggested. Illustrations show how to cater for differential movement of foundations. 8 pages.