Total design heat load as well as the approximate method based on the thermal characteristics of the building were determined in accordance with the PN-EN 12831 standard for 84 randomly selected single-family residential buildings located in rural areas. Calculations were carried out to assess the accuracy of the approximate method for estimating the heating power demand for single-family houses, as compared to the detailed method, and allowed to determine the Mean Absolute Error (MAE) which is 1.1 kW and the Mean Absolute Percentage Error MAPE which is approx. 10% for the analyzed group of buildings. The resulting error values form grounds to the declaration that in absence of construction documentation the approximate method provides good results when estimating the demand for power for heating buildings.