A development at 83 Queens Road in Melbourne, Australia, required construction of a three level basement. The normal geotechnical constraints of excavating adjacent to an existing relatively new building were challenging enough for the complicated soils profile occurring at the site. An additional significant challenge faced by the designers was the requirement to excavate underneath the entire footprint of an existing heritage listed building, known locally as “the Mansion”, in the centre of the site. Maintaining the structural integrity of that building was afforded the highest priority during the 10 m excavation. This paper describes geotechnical aspects of the analysis, design and construction of the secant retention system adjacent to the new building and support of the Mansion enabling the 10 m excavation to proceed without causing damage to the structure. The secant wall comprised hard-soft interlocking piles constructed using CFA methods. CFA piles along the length of the Mansion provided short term support with the building load transferred to the piles by large steel beams jacked under the building. Survey results confirmed that movements of the retaining wall and also the load bearing piles were minimal and well within design expectations.