Hi ,
I would absolutely recommend using an external program to model your assets in. There are known issues with BSP especially when using a lot of them in a scene that would probably lead to more headaches than not. If you’re having trouble with modeling you can always ask questions here and everyone will gladly help guide you. 
Something to consider though. And I do recommend the use of BSPs here would be that you could do a box brush BSP and get the general scale of your house > Convert that to a static mesh > export from UE4 to FBX > import into your modeling program for size reference.
This, in my opinion, is a good way to keep the scale there for your reference. It’ll give you a good starting point.
If you have any questions let us know!