Foundation and the disappearing walls

[rhinosaurus]

I've done a search but I'm clearly not using it properly, so sorry if this thread is already in existence.

What is the proper method for building foundation so that you don't destroy existing walls?

For example, I want to extend the building, but not tear down the walls of the cell block I've made that is adjacent to the new part I'm building.

I've managed it a couple times but I don't understand what I did differently. Half the time I end up having to rebuild the walls on the interior after the foundation is done.

[VoiD88]

Do a search next time before posting please
http://forums.introversion.co.uk/prisonarchitect/viewtopic.php?p=16055&highlight=#16055