Is this a fragmented sentence . Since you no longer maintain an office downtown for working on this kind of project?

So many Words, but not a real sentence, fragmented or not. If you remove the Word "since", it will make it a real sentence:

You no longer maintain an office downtown for working on this kind of project.

The subject in now "you" and the verb is "maintain".

By using the Word "since" you need to answer what can or can't happen due to the phrase "you no longer maintain...".

'Since' used as a subordinating conjunction means 'because' in the above sentence there needs to be some result following. Since ................+................(result).

So the sentence - with since - is not complete.