Modular Level Design - Do Inside Unity or 3D Software?

I'm using Unity indie version.

I am looking forward to learn from you, some of your experience regarding level design.

Initially, I plan to model small model and then rearrange them inside Unity editor.

However, if my understanding is correct regarding draw calls, this method will increase my draw calls which then will decrease the performance level of the game.

Then I thought, is it even better if I model small models in 3D software and re-importing these assets, assembling them inside the 3D software instead of Unity. (Still using modular modelling concept here.)

For example.
Instead of rearranging walls, windows, and door inside Unity, I models and rearranging this inside 3D software and export the whole building as one big model.

Then I only have to rearranging buildings inside Unity instead of everything from walls, windows and doors inside Unity from a smaller modular models.

What is your thoughts?

 

You could do this in a model editor, but there are also assets like ProBuilder that allow you to do this in Unity. You can also save a lot on draw calls by batching your materials properly.

 

Wow, that's a nice addon.