Welcome to MH's forum and a bunch of good people who know a lot more than I do. I have specialized in residential and have worked in energy management to know that keyiing in to local and state energy codes are a major prerequisite to setting up load minimums for taking both lighting and HVAC into consideration. Even with a shell design, principle enforcement of both spec and prescriptive energy minimums come from the driving Code enforcement requirements at the building department. If the structure is made of heavilly glazed wall fenistrations, you must include the conditioning that requires either passive or active environment compensation. So starting at the permit specification stage and the building department will be a great source of expertise in electrical loading design. (i.e. Regional zone minimums such as type of structure, watts per sf lighting, btu's per sf heating, etc. vary determinatly by climate.) I have not found any tried and true rule of thumb without starting at the planning department.
rbj, Seattle...Safety is a Professional Courtesy.