The changes in Spanish legislation on energy efficiency seek to set the basis for achieving European energy and climate goals, especially insofar as decreasing primary energy consumption and reducing CO2 emissions. This has led to positive changes in energy in the building sector from traditional construction methods which did not consider energy efficiency (e.g., did not include envelope insulation). This study analyzed 342 residential buildings built from 1981 to 2015 and characterized their parameters (areas, orientation, number of stories, etc.), envelopes, domestic hot water, and heating and air-conditioning systems. Based on this characterization, a model single-family dwelling and another model multifamily dwelling were designed to evaluate their energy efficiency as a function of changes in construction and their cost in southeast Spain. The main results were that recently built buildings (built after 2006) have included domestic hot water from solar sources, insulating materials in their envelopes and have considerably improved construction systems, due to the requirements of current legislation. Fruit of these improvements is a drop in CO2 emissions of up to 82%, according to calculations done with official programmes. From an economic perspective, these changes have raised construction costs by 11% in multifamily dwellings and 17% in single-family dwellings. (C) 2015 Elsevier BM. All rights reserved.