Old Building

was build in and with imperials.
Now I have to measure and redraw all in meters.
I have to improvise and understand how my ancestors do this.

do you still need some models to be presented in metric? Why?

Because the new plans made by me have to be in metric system.
Not all around me know what mean 1" or 2 3/8".
The plans has to be populated with the exiting furniture.
I have to represent it in metric system even if them are build under imperials (like I K E A furniture in the pictures).

objects are ALWAYS exported to an .obj file in metric

Of course but I talk about the REAL objects not the virtual ones which have to be represented in plan.

Sweet Home 3D (and any other application) can only work with one system at a time

SH3D work always ONLY in cm.
What we see in plan are transformations of cm in different measurement unit.
I think it is not a problem to see two (in bracket) in place of one.