are permits needed for renovation in 20 unit bld?

It's not the size of the building, it's the type of work done that determines the permits required. You can look up info on specific buildings at http://www.nyc.gov/html/dob/html/home/home.shtml.

Yes. Definitely. There is a very limited scope of work to what you can do WITHOUT a permit (ie: replace fixtures IN PLACE, cabinetry in the kitchen, etc). Obviously there is plumbing work being done (moving an entire bathroom), probably some electrical, probably new walls or some type of layout re-arrangement (perhaps not - but a gut is quite extensive).

What do you mean by legally? Was the work already done? Are you looking for proof of proper filing?

yes, work has been done and since then there it leaks into my apartment on occasion . I am out 3k in dames thus far over a 4 year period. I want to know if in fact the work was done properly and professionally. My landlord has not been cooperative with the issue and jus keeps painting the ceiling. i guess proof of filing is what i am looking for.
many thanks for help!

Check the on line NYC permits database

http://a810-bisweb.nyc.gov/bisweb/bispi00.jsp

thank you. there is nothing posted on that site about anything being done on that unit.