Ralph Erskine Arkitektkontor’s project for a 2,000-unit housing estate in Byker, Newcastle-upon-Tyne (1969–82) was the result of an unusually long and complex design process. The critical literature highlights Erskine’s humanist position and socialist inclination, but the operations of the project have never been explained in detail. Based on literature and archival research, this essay examines Erskine’s response to the challenges of the commission. As a reaction against the modernist ideals of efficiency and homogeneity, the project sought to absorb the pre-existing social structure, approximating the image of a medieval city, with its perimeter wall, and its houses and alleys of varied scales and irregular patterns. The term isomorphism – a structure-preserving map – is proposed here to capture the procedures that the architects deployed to give the project a theoretical and formal grounding, establishing continuities with the past that would soften the impact of the radical physical transformation imposed on the site. The ambiguous spatial and material qualities that characterise the project for Byker were fundamental in creating the conditions of reception that made this process viable.