Hans

I'm a bit confused about what you are recommending, perhaps I don't understand the terminology. Your statement (1) suggested that the Software Centre was used to install the program, whereas (2) suggested that you just downloaded the program.

I never use the Java Web Start version,

I couldn't make that work so don't use it.

but always the installed version (1)

When I use the Ubuntu Software Centre or your link it doesn't work.

I then download and unzip SH3D in its own directory (2)

As mentioned above, this is what I am using, although I haven't unzipped the .jar file.

I can then start the program by double-clicking SweetHome3D, a shell script of 771 bytes.

I could not find a shell script. I use
java -Dcom.eteks.sweethome3d -jar /home/user/Documents/SweetHome3D/SweetHome3D-3.3.jar

I hope this clarifies my confusion - if you see what I mean!
Chris