Expansive soils are soils which experience volume change when soil moisture changes. All clay soils experience some volume changes from moisture changes, these volume changes are cyclic type movements and can cause severe damage to buildings.
Soil moisture may change due to a number of reasons such as seasonal variation, trees, leaking services etc. It is recommended large trees are never planted close to the house.
The minimum depth required for foundations recommended in NZS3604:2011 is 200mm below cleared ground level.
However this minimum recommended depth is more suited for non-expansive soils. The majority of soils in Auckland fall in the category of Class M moderately expansive soils. To reduce the risk of seasonal foundation movements many geotechnical consultancies are now recommending a minimum foundation depth of 600mm below cleared ground levels, into “good ground.”
There are laboratory test methods to detect how expansive a soil is, which are quick, relatively inexpensive to carry out and should be carried out on a more regular basis to estimate characteristic surface movements.
I would recommend prior to putting the foundation and building plans in for building consent that a geotechnical engineer is consulted about the recommended soil expansivity of the site for design purposes. It is a nuisance to discover the depth of the foundations are not deep enough and need to be deepened last minute before concrete arrives.