Hi Adam,
You can render images out of Unreal on an Alpha channel.
In your project settings, search for “Alpha” and set the following setting:
In your level, you will need to construct your render scene where only the Building is visible.
If you have only have the Building and lights in the level, that should be fine. If you have a background behind it, or other objects, or an HDRI, you will need to set a couple settings on all of those meshes. Set “Visible” to Off, and “Affect Indirect Lighting While Hidden” to On.
When you render in this way, the building should be on an Alpha channel in the render.
You will need to use an image type that supports Alpha channels such as PNG or EXR.