This may be a dumb question, however when you don't know the answer, what can you do but ask? If I purchase additional 20GB storage space, am I getting an additional 15GB space (on top of my 5GB free storage) OR do I retain my 5GB Free storage and have a total of 25GB?
Indeed, the storage is summed up, you will get 25 GB.
If I understand it correctly, you will get 20 GB additionial storage, so you will have 25 GB storage.