To learn how to calculate the area using the BOMA Retail Standard, you should purchase a copy of the official document from BOMA. There are too many intricacies and variables to consider. The purpose of this forum is to answer specific questions and to provide interpretations of the standards.

Having said that, I will answer some of your questions and offer some advice.

The retail standard does measure to the outside face of exterior walls.

If the exterior wall thicknesses of the basement cannot be determined, use the building line determined on the ground floor as a basis.

Stairs are generally part of the common area but may also sometimes be considered an occupant void.

The BOMA Office standard would differ in a couple of key ways. In the Retail Standard, the measure line is to the outside face of exterior walls, and under the Office Standard, the measure line is to the vertically dominant inside finished surface of exterior walls (there are some exceptions to this). Also, there is no gross-up applied for common areas under the retail standard, whereas in the office standard, your tenant would pay for 100% of all common areas (even under a single tenant scenario).

In this case, the BOMA Office Standard will probably yield a larger rentable area.

You might want to consider using the BOMA Gross Areas Standard, which is frequently adopted in single tenant buildings and will essentially include the entire building, measured to the outside face of exterior walls without any deductions.