I found out how to demolish buildings that have already been built.
Go to the building you are trying to demolish. If it is a multi-story building start at the highest floor and select an area of floor and use the design tool to remove some of the flooring and wait for the flooring to be removed.
Pause the game.
In the same spot you removed the floor add flooring of the same color back into the same spot. Once you have done this you should notice it will have the original project (building) number for that section you are working on.
While the game is paused and the floor is not finished being rebuilt select the remove button. Your workers should start to do a full dismantle.
I just tried this on three different saves and on a multi-story building with a roof and all and it worked every time.
Hope this helps.