Try these steps,
1. Make/design your model at any 3d-modeling software
2. Apply transparency texture(color mode =black&white *Black= Renderable *White = Transparent ) to it. And use *.png only to work with SweetHome3D.
3. Align UVW mapping.
4. Export as .obj(wavefront OBJ) or .3ds (check Transparency texture must be added in the same folder ).
5. Import exported obj/3ds file into sketch-up/any other 3D model viewing application to check transparency map working or not.If transparency does not load you failed to create model correctly(Need re create your model).
*Advanced user TIP
If your exporting model as .obj format then open .mtl file comes with it in Notepad
and check under glass material name consist "Ni" must have a value(in between min 0.001& 10 max) for glass material
and "d" value in between '1' and '0' for transparent(1= no transparency,0= Fully transparent/invisible).
6. If transparency working correctly then Import 3d model as furniture in Sweet-home-3D.
7. Select imported furniture, go to Furniture -> Modify in main menu
8. Select materials radio button in color& texture, then click Modify
9. Select model material(your transparent object material) then select Texture radio button and click browse button.
10. In the new window click Import button & browse, open your texture map and click OK for all windows.
*10. If object don't have a texture file, fully transparent like glass
then select color radio button and select white color in modify Furniture material window.
11. Now render your image.
If the default glass rendering may be look like this.
Then go to modify & pick original color from the color pallet left to the current color using color picker tool as shown below &press ok, then render.
TIP.
If you want to hide(invisible in render) a particular part of an object then use my texture. as your material.
(This file consist zero color pixel). [:))]
