This paper addresses the effect of a winter garden in a passive detached house on its energy parameters. To carry out the study, it was required to design dwelling building compliant with passive construction standards. The building was intended for construction in north-western Poland. Compliance with the requirements of passive construction standards allowed for the use of a buffer zone in the form of a winter garden. 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 avoid overheating and tropical temperatures inside the garden in the summer. Also helping this purpose are white venetian blinds used on the garden's vertical outside walls and colourful roof marquise. A winter garden was designed in the form of a rectangle of 4.36 m x 3.03 m with a pent roof. Its design is based on a mullion and transom facade system. It consists of 50 mm wide profiles and double-glazed windows. Calculations related to the energy balance were performed for the two adopted variants. 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 heating and ventilation in the heated area during the year by more than 30%. In 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 building's energy balance in climate of north part of Central Europa.