After a brief presentation of the program éco21 and the framework of its monitoring and evaluation, this paper addresses the evaluation methodologies pertaining to two of the ten subprograms constituting éco21: Low Income Household (éco-sociale) and Building Communal Areas (communs d'immeubles). For these two subprograms, different options of enhanced engineering estimations—that will be used with a sample of projects—will enable to refine simple engineering estimations. (The terminology is inspired from the Directive 2006/32/EC) The aim is to drop the enhanced engineering methods later in favor of the “improved” simple engineering estimations that are more efficient in time and cost. Billing analysis will be used systematically to double check the estimates and to evaluate the lifetime of the actions. First evaluation experiences: the results of the very first evaluation experiences for the two subprograms (éco-sociale and communs d'immeubles) are presented. First feedback to program managers: the first pilot project of éco-sociale showed that the electrical power of the lights bulbs, which were replaced with more efficient ones, has an average value that is lower than the one expected before the implementation of the pilot project. However, for Building Communal Areas, the results from a sample of projects implemented during 2008 and 2009 show that the actual savings per project are higher than the expected savings.