Hi Piotrass, You could try using Extrude Mesh Volume . Here is an video example I made using roof surfaces and a foundation shape similar to those in your image. All items are extruded to the same elevation. This created a single mesh volume from the…

If you follow the method I suggested you end up with a "water tight" mesh volume. This can then be converted to a smart solid via Convert to Solid . You can also simplify the mesh first with the Cleanup Mesh tool.