The energy rehabilitation of listed buildings guaranteeing heritage values but allowing a use that contributes to their conservation, supposes a challenge with the need of a holistic approach. Buildings of the Modern Movement, many of which are registered as DOCOMOMO (DOcumentation and COnservation of buildings, sites and neighbourhoods of the MOdern Movement), are a particular case since many of them are not yet listed or are under unclear requirements. This paper explores the inclusion of a cost-effective methodology as part of the decision-making in the energy rehabilitation of these DOCOMOMO buildings, applying it to the case study of an office building located in the north of Spain. Different scenarios were studied balancing cost-effectiveness, energy efficiency and rehabilitation requirements. In this case study, the analysis may allow policymakers to have supportive arguments to subsidize certain elements (as e.g., steel frames), or allow the use of alternative options with similar aesthetic characteristics, but at a much lower cost. This second option will constitute the unique cost-effective scenario, with energy savings of between 25.36 % and 38.8 %. The inclusion of a cost-effective methodology as part of the mechanics for decision-making in the energy refurbishment of DOCOMOMO buildings permits the optimisation of the intervention guaranteeing their use and the conservation of heritage values.