his paper presents a design strategy applied to the rehabilitation of housing in critical context. Florence House is an old abandoned hospital that is now spontaneously occupied by poorest inhabitants of the city of Johannesburg without any formal planning. How architects can be involved in the improvement of such a extreme environment? In order to do what ? Following which principles? The strategy advocated here is based on some simple theoretical and operational principles that are : a precise and methodical observation of context, the economy of means, the thought of recycling, the use of informal institutions, recognition of all the skills existing into the site. In this work, the architect acts more like an active researcher less rather than pure expert.