In that case It is simpler to change a geometry in FreeCAD than in sh3d.

Correct. Don't compare Sweet Home 3D with a CAD application. It has a totally different purpose. Sweet Home 3D is first and foremost an interior design program. Over the years many features were added and it now is also a great program to design a house. Don't expect any specific features that a CAD program has, especially not accuracy. The smallest unit is 1mm which is small enough for any building but is pathetic for a CAD program.
You will have to decide for yourself if Sweet Home 3D is the correct software for your purpose. If it is, you will have to learn how it works without expecting that it works the same as a CAD program.