This paper addresses the effect of a winter garden in a passive detached house on its energy parameters. In order to carry out the study, it was necessary to design a dwelling building compliant with the passive construction standards. The house was designed as a detached two-storey building with a pitched roof and no basement, constructed using traditional brick structure of double-layer walls. The building was intended for construction in north-western Poland, Central Europe, in Pomerania Region. Compliance with the requirements of passive building standards allowed for the use of a buffer zone in the form of a winter garden. Within the project, the garden was designed as an enclosed unconditioned area located at the southern side of the living room. In the winter garden, there are ventilation openings and air inlets intended stay closed during the autumn-winter season. However, in the summer months, they are necessarily opened to provide air circulation. This solution will help to avoid overheating and achieving tropical temperatures inside the garden in the summer. Additionally, there are white venetian blinds used on the garden’s vertical outside walls and colourful roof marquise. For the purpose of the study, a winter garden was designed in the form of a 4.36 m x 3.03 m rectangle with a pent roof and slope inclination of 25°. Its design is based on a mullion and transom facade system. It consists of 50 mm wide profiles and double-glazed windows. The calculations related to the energy balance were performed for the two adopted variants. The heat gains and losses as well as dynamic parameters and heat demand were evaluated. It was found that the winter garden has no significant influence on the temperature conditions in the building. This applies primarily to a small part of the facade to which it is adjacent. In addition, the effect of the adopted monthly calculation methodology on the obtained parameters was shown. In general, the addition of a winter garden to the building reduced the overall demand for space heating and ventilation in the heated area during the year by more than 30%. In the case of the second variant, the duration of the heating period was also reduced by almost 230 hours. This also resulted in lower annual primary, final and usable energy demand values. Finally, it was demonstrated that a winter garden has a positive effect on the energy balance of a building in climate of north part of Central Europe.