What do I do if only a section of the wall is a party wall?

If only a section of the wall is a party wall, for instance, if there is a garage behind half the wall but not the other half, you can split the wall using the blue handles . 

 Simply drag and drop the blue handle to the point where the wall needs to be split. 

 You can then select whether the other side of this wall is a heated or unheated space. In the example, you would select one half of the wall as unheated space and the other as outside .