Best way of designng an indoor facility/building level

Hello, I got a quick and pretty small question. So I'm pretty "new" in game development.

I was wondering, what the most efficient way of creating/designing an indoor facility level:

a) Using Unity-Meshes (made with Polybrush and/or planes) as walls, floors and ceilings and make static props using an external 3D Modellng Software (i.e. Blender) b) Modelling the entire facility, including static props, in an external 3D Modelling Software

So, I'm little new to indoor-facility design, I mean only one room isn't pretty hard - I already did that once - but an entire facility is just something different, atleast it fells for me. I'd appreciate any tips!