Architect for Consultation re Codes

My wife and I have had off- and on-again negotiations about acquiring basement space adjacent to the lower level of our apartment. Our apartment (a duplex) was redone years before we bought it, apparently on the sly, and doesn’t have a C of O.

We’d always figured that if we expanded the apartment we’d take care of the C of O problem as part of that process. But before embarking on such a project, I’d like to get a better sense of the possible code/permit problems we might face, from making the earlier work legit to showstoppers such as the possibility that our current downstairs doesn’t meet the definition of an English basement, and we should lay low in our dubiously legal living space instead of trying to expand it. (I think we’re fine, but I’m no expert.)

Does anyone have a recommendation for an architect who could do such consulting? And any idea what such a consultation might cost? Any advice very much appreciated!