In the energy conservation standards for residential buildings, the standard value for annual heating / cooling load is stipulated for each territory. The division of territory is determined based on the heating degree-days, which have high correlation with heating load. This paper clarifies the effect of altitude on this area classification, and examines a new area classification that reflects the altitude at an arbitrary point. The obtained results are as follows: (1) Heating / cooling degree-days have a close correlation with latitude and altitude. (2) By utilizing the relation between ambient temperature and altitude, the heating / cooling degree-days at sea level (0 m) without the effect of altitude can be obtained. (3) On the basis of the heating / cooling degree-day at sea level, the heating / cooling degree-days at an arbitrary point can be estimated. (4) By employing the estimated values of the heating / cooling degree-days, a new area classification corresponding to the trends in annual heating / cooling load at an arbitrary point can be performed.