The housing sector in the Netherlands is responsible for a significant fraction of primary energy use and CO2 emissions. Great energy conservation opportunities are to be found in the existing housing stock, especially in large renovation projects on existing sites. Energy conservation savings of up to 90% are technically feasible. Despite this, there is little empirical evidence available about processes that influence the achievement of energy conservation goals in such locations. Moreover, no systematic, bottom-up research on the matter is available. This paper attempts to answer questions about the factors – size, direction and significance – that explain variation in the degree of energy conservation. Four main propositions were tested, comprising the following variables: actor characteristics, policy instruments, interorganizational collaboration and context. The study used a comparative research design. Data were collected from eleven existing housing sites where renovation projects had been executed, involving 70 personal interviews, a survey, and the collection of project documents. A mixed methods approach was applied for data analysis. The results show that interorganizational, collaborative efforts, policy instruments and the presence of wealthy housing associations have a positive influence on energy conservation outcomes. The mean energy conservation was slightly less than 40%, and outcomes varied between 26.5% and 69.8%. Strikingly, planning does not have a beneficial influence and the actual outcome is lower than predicted. The results are useful for national and local government policy makers, as they clearly argue that ambitious policy goals should be tempered.