The aim of this work is to study the possibilities for energy-conscious and environmental-friendly retrofitting of single-family houses in Sweden and Hungary. According to the climate, energetic, architectural characteristics of the countries, the following options were studied in more detail: additional insulation of buildings, passive solar gain systems and active utilization of biomass and solar energy. We choose two type houses in Hungary and Sweden to apply our knowledge in real retrofit situations. The engineering and architectural problems were examined based on energy, environment and economics and suggestions for retrofitting were made. The chosen type houses were named as “Kecel” and Pecel” for the Hungarian houses and Building A and B for the Swedish ones. In the original case, the Hungarian houses “Kecel” and “Pecel” had a heating demand of 36310 and 40195 kWh, respectively. The second house had a useful floor area of almost twice as big as that of the first one, however, due to the stricter norms and more careful choice of materials the heating demand was only slightly higher. The Swedish Building A and Building B had a yearly heating demand of 26670 and 17903 kWh, respectively. Building B similarly to “Pecel” was built later according to the standards and had bigger floor area. It is worth noticing that the Hungarian houses had a demand of almost twice as big as the Swedish houses, even though they were also not so well insulated. The first step that we considered was to add insulation to the boundary structures in every house. As it had been expected, the additional insulation was the most cost-efficient in case of the worst insulated structures and buildings. The heating demand of every house reduced significantly, however the most spectacular was the reduction in case of type house “Kecel”: from 36,310 to 36,638 kWh. In general, we can say that additional insulation of the houses should be the first step to consider. Our economical calculations also proved this. Especially if we consider the escalation of the fuel prices, avoiding the waste of energy becomes very important. As a next step, we considered the solar retrofit of the houses with solar domestic hot water and combi-systems. Both in Sweden and Hungary there is a great potential for solar applications. However, investing in solar is not always worth from the economical point of view. We optimised the system sizes according to the specific investment costs and made economical analysis. However, we did not get any rational payback periods for the systems applying average Swedish prices and current energy prices. Hopefully, in the future, the system prices are going to decrease due to improved designs and mass production. On the other hand, due to different reasons, the energy prices are probably going to increase in the near future in both countries. These factors might influence the implementation of solar energy positively.