Lien against a building instead of an apartment owner?

Mechanic's liens are always filed as a lien against the "real property" (i.e. the building) as security which is why a mechanic's lien foreclosure lawsuit names the Owner as a party defendant. In other words, if you cannot collect against the contractor, subcontrator (or owner as the case may be) for breach of contract, you can foreclose on the property. Residential liens are good for four months from the last date of furnishing labor, materials or equipment and commercial is eight months. If you need an experienced construction lawyer, or simply to just properly have a lien filed, please feel free to reach out. 

A "condop" is a unique hybrid that was created to address a tax law that no longer exists. A condop is generally a condominium overall, but one of the condominium units consists of a number of apartments jointly owned in cooperative form. Accordingly, in these buildings, it's important to know what side of the house you worked on, and whether it's a true condo, a true co-op, or a co-op of a condo unit. The lien process is different for each, but it is doubtful that you can get the building under any scenario. However, if the unit is true condo, you can lien it as real property. If the unit is true co-op, you can lien the proprietary leasehold interest. Under either scenario, condo and co-op units are considered single family homes, thus requiring the assertion of the lien within four months from your last work on the project. The lien is good for one year, and you can bring special proceedings in court to extend it for each of three successive years. 

I was under the impression that if the co-op consented to the work being performed on an individual unit, a mechanic’s lien filed against the building as a whole may be enforceable.

That is correct, unless you have the specific consent of that owner (there, the co-op) to have performed your work, you cannot lien the building. However, you can assert liens against lesser estates in property, which include leasehold interests. Keep in mind that although the co-op is a corporation and the "unit owners" are mere shareholders in that corporation, what gives individuals the right to live in a particular apartment is a proprietary lease "married" to those shares. You can assert a lien against, and foreclose upon, that lease and end up with the effective ownership of the leasehold. This is often sufficient to have the owner find his or her wallet and issue payment. 

The answer depends on the building and who retained you. If it is a condo and the unit owner retained you, then you should lien the apartment unit. If you were retained by the landlord/managing agent to do work in a unit, in a coop, lien the building and name the managing agent as well as the coop corporation.. If you were retained by the building to do work in a condo unit, lien the unit and name both the unit owner and the condo manager and the condo association. Generally liens must be filed adn served within 4 months of the last for a home and 8 months for retail or commercial.  

if you send more details on who retained you, and the type of buldingi will give you more specific answer. 

In this particular building the owner of the apartment had to get approval for the work to be done plus review my insurance documents to make sure I meet their insurance requirements. Wouldn't that be consent?

Not necessarily. You are correct that there is a typical requirement of procuring additional insurance for the co-op and having that insurance appproved by the co-op. However, courts are loathe to equate that with consent sufficient to render the co-op itself liable for sn improvement undertaken by one of its unit owners. I think if you try to assert a mechanics lien against the building you will be buying yourself an expensive fight that you would likely will not win. However, as I previously mentioned, you can assert a mechanics lien against the lease, which I think will be sufficient to get you where you need to be.... that is, paid.

Not necessarily. You are correct that there is a typical requirement of procuring additional insurance for the co-op and having that insurance appproved by the co-op. However, courts are loathe to equate that with consent sufficient to render the co-op itself liable for sn improvement undertaken by one of its unit owners. I think if you try to assert a mechanics lien against the building you will be buying yourself an expensive fight that you would likely will not win. However, as I previously mentioned, you can assert a mechanics lien against the lease, which I think will be sufficient to get you where you need to be.... that is, paid.