Sorry I thought it was already in the thread
I registered my rep. office at my girlfriend's address. In general, you can register your business (rep. office, limited company, branch office) at any address, as long as "your work matches your surroundings". So no scooter repair in the flat on the 5th floor, but sitting in front of the computer and sending mails is ok anywhere. Which is what you are supposed to do as a rep.
The only thing I needed was the latest house tax payment slip of the place, other than that it was smooth sailing. Somebody from the CLA came by to see me and my office, so I gave him something nice to look at.
As stated in the first post, this might imply property tax changes for the place, but so far nobody told me anything . Also, outside of the scope of this thread, if you have a real limited company or branch office, and you want to deduct office cost as business expense, you should register at a commercial place.
This info is in the updated Llary post (somewhat), in the book "How to do business in Taiwan" and from the MOEA itself.
UPDATE: This is the website of the book I mentioned startabusinessintaiwan.tw/ They had some ads here as well