A cold water storage cistern must be:

A cold water storage cistern must be fully supported across the base and should be 150 mm larger than the cistern itself to ensure stability and even weight distribution. This support helps to prevent any undue stress on the structure of the cistern, which can lead to cracking or failure over time. A larger base also provides a buffer zone that can accommodate movement or settling, which is particularly important in areas with soil expansion or contraction due to moisture changes.

Supporting the cistern only at the corners, as suggested in some other options, may lead to an uneven distribution of weight, increasing the risk of structural failure. A base with overhang can also create the potential for instability if the overhanging part is not adequately supported. Installing the cistern below ground, while it may provide some stability, is not a standard requirement and can complicate access for maintenance and repairs.

Overall, ensuring that the cistern is fully supported and larger in base dimensions enhances safety and longevity, which is critical in plumbing installations.