Heating load calculation in general for HVAC system design ?

To calculate the Load Calculation one should consider the follow aspects.

1. Building Orientation
2. Use of Space Ex. for Hospital, School or Residencial Etc
3. Physical Dimension of the Space
4. Ceiling Height
5. Construction mAterial Ex: Type of concrete used, partition wall, insulation, paint etc
6. Window glass type 
7. Lighting Accessories
8. Electronic Appliances 

At least you have considered the above-mentioned aspects to calculate the load calculation.

Please refer to the ASHRAE hand book or basic carrier hand book which elaborate everything

Generally, heat load calculation is a process of calculating all heat loads generated from outside and inside the building. It considers, building orientation, climatic information, building intent or occupancy type. I usually refer to architectural drawings for building information needed. While doing the calculation building materials U-Values plays a major role. It is more reliable if  both manual and software generated calculations are done.