Expand the Mezzanine?

It can be done but the mezzanine area has to be non livable area and has to be a storage space. Remember several years back about the Brooklyn centric architect Scarano who use to abuse his FAR with illegal duplexes?

But, if your going to hire an architect for the proposed idea, I'm sure they tell you the cans and can'ts.

I'm not an expert by any means. But I do own and have filed plans to renovate an apartment with an L-shaped "mezzanine" that houses a bedroom, den, and bathroom, and am happy to share my experience.

Usually, a mezzanine is a mezzanine for a reason. Different building codes apply to mezzanine spaces. You can't, generally, turn a mezzanine into a true, legal building story.

It also seems, based on a lot of research, that mezzanine spaces run much higher risk of not being fully compliant with code. Clever architects have pushed the limits or made post-CO modifications that leave many mezzanines of questionable legality. There was the high-profile Scarano scandal, but I've also come across a lot of lower profile situations, especially in my neighborhood of Chelsea, where architects who rehabbed buildings in the '70s did all kinds of crazy sh*t to get around the legal three- or four-story restrictions.

Lucky me, I live in one of the spaces :). I did not realize my upper floor was a mezzanine and not a story until I went to renovate. In every professional appraisal I've seen, the building is listed as four stories (sometimes five - since we have an above ground basement), neverminding that we're by zoning and fire code only allowed three.

When we submitted our renovation plans to the DOB, the city was very interested in this mezzanine (which doesn't conform to any of the requirements of a mezzanine, but also can't be a full floor). In the end, we could prove the space had existed this way for 40 years, and the DOB turned a blind eye. We were even allowed to make small modifications to the floorplan of it. Expanding, though, was out of the question. Residential mezzanines are supposed to be something like 1/3 of the space, and ours was already at 75% of the downstairs floor area.

My guess is that you're out of luck, but you can always consult an architect and expediter/building code specialist to be sure.