Abstract: While researchers have developed rigorous practices for offline housing audits to enforce the US Fair Housing Act, the online world lacks similar practices. In this work we lay out principles for developing and performing online fairness audits. We demonstrate a controlled sock-puppet audit technique for building online profiles associated with a specific demographic profile or intersection of profiles, and describe the requirements to train and verify profiles of other demographics. We also present two audits using these sock-puppet profiles. The first audit explores the number and content of housing-related ads served to a user. The second compares the ordering of personalized recommendations on major housing and real-estate sites. We examine whether the results of each of these audits exhibit indirect discrimination: whether there is correlation between the content served and users' protected features, even if the system does not know or use these features explicitly. Our results show differential treatment in the number and type of housing ads served based on the user's race, as well as bias in property recommendations based on the user's gender. We believe this framework provides a compelling foundation for further exploration of housing fairness online.