Hey LordNorthern!
First of all, if you want to create interiors for Top Down City then definitely check out
https://www.unrealengine.com/content/47d8fb4f1e2b40b590369065f0716fb3
screenshots to understand what exactly you want to create.
I don’t have Top down city, but as far as I can tell, you can simply export house asset(if it is not against the rules of using this asset) as fbx and import into Blender.
Then you simply move beams, walls etc to make interior wall and remove other building assets.