Decided to dig a hole in the garage

[NZCSLBMW]

In New Zealand the pit would be made as though you are building cantilever retaining walls, and requires steel reinforcing in the floor and up the sides. Shuttered walls with solid poured waterproof concrete As if blocks were laid it would require tanking behind and difficult in a renovation. In the poured concrete a water proofing agent from a Hydrophilic (is a soluble crystiline system) or hydrophobic (which a fatty acid system) of blocking the pours of the concrete. For a belts and braces waterproofing the inside can be sand cement rendered, with an additive from Sika Products and plastered. An alternative is a paint on membrane from Mapei.