The most significant increase in construction volumes in Latvia was registered in the time period from 2005 to the middle of 2008. Many large suburban areas around Riga were landscaped for building single-family-type houses. People have been using these properties for 12–15 years, and now, the challenge for the inhabitants is to find the most efficient way to maintain a high level of living comfort. Deteriorating systems require investments, so it is in the interest of owners to ensure that the benefits of such investments are maximized and that energy consumption is as low as possible. In this study, the authors simulated various scenarios where HVAC system parameters were changed, and the thermal performance of building structures was improved. Annual energy consumption, indoor quality and thermal comfort were analyzed. The importance of this work is justified by the need to realize and define the energy efficiency levels of existing single-family houses and demonstrate the amount of investment required to move closer to established energy efficiency targets.