Addition of a Roof Deck and master suite

You need to talk with an architect.

She'll ask whether the building is a co-op or a condo and what kind of rights you have to the roof.

I.e., if a co-op, the co-op would sell you additional shares for the additional space. If a condo, whether the roof is a limited common element appurtenant to your space, or a general common element owned jointly by all the unit owners. If the latter, then the condo would have to agree to license the space to you.

That answers half the question of whether you can build up there at all. The other half is, considering the existing square footage of the building, whether the city will allow the building to get bigger.

One way to start would be to go up there and look around at neighboring buildings. If you see other additions, then look them up at the Department of Buildings, find out who the architect was, and talk with her about what was involved.

To take another tack, think about why there's nothing up there already. If a co-op, did the previous shareholder even try? If a condo, the sponsor would've developed and sold every buildable square foot already.

You didn't describe here or before (http://streeteasy.com/talk/discussion/37926-converting-loft-2-bedroom) the neighborhood and building. If you'll do that you'll give people more to go on.

Thanks for the comment. The building is a seven story loft in the flatiron. built in the 1920s.

I added a 300 square foot roof deck addition to my apartment in Chelsea (no interior square footage other than the bulkhead). The project is not for the faint of heart.

You'll need an architect to do a conceptual plan, and likely a structural engineer to do a weight load calculation on the roof. This will cost a few thousand dollars and likely require cutting a few large holes in your ceiling. Based on the age of your building, you're likely going to need some type of reinforcement -- anything involving steel is likely to be very expensive. Don't assume, based on neighbors, that you'll be OK. Building codes have changed, and what was possible 15-20 years ago without reinforcement is no longer possible today. (We found out the hard way.)

Budget in asbestos abatement, too. Chances are at some point in time some material that might possibly be asbestos was used in your roof. I had to go through this even with a signed affidavit that no asbestos-containing materials were used in the construction of my building, due to a "suspicious" test result. (I could have fought it, but it would have pushed my project from summer to winter.)

You'll need an appraisal done on the space -- expect to pay a few thousand. There are three big firms that do this, Miller Samuel, Vanderbilt, and Mitchell Maxwell & Jackson. If a co-op, you'll need to pay for the co-op attorney to obtain a no-action letter from the attorney general, and then pay for your own attorney and the co-op's attorney to close on the shares. The process is more complicated if you don't have enough shares outstanding.

If you get to the construction phase, that's where the real fun begins.

Materials will likely need to be hoisted or even craned up. Expect costs to be 50-75% higher, and budget for things like crane rental and street blocking permits. Expect Murphy's law to apply. The day we installed a steel beam, there was a freak, unforecasted thunderstorm that flooded our unit and the one below. The day we craned up the bulkhead to the open roof was the day of the freak October snowstorm; we were shoveling snow inside the apartment. Related to the above, don't figure on being able to live there while construction is going on.

With all of the various costs -- especially the endless professional fees -- the cost of adding the space was probably more expensive than fair market value for buying the finished space. And that was with a dirt-cheap price to buy the roof rights, given many limitations to any unit other than ours accessing the space. Once the roof is open, there's no going back. The project has to get finished somehow, even if the cost triples.

We had looked for an apartment for three years, knew there was nothing like the space we created, and forged ahead. But, wow, was it expensive, all-consuming, and stressful. I would never do it again and caution against it unless you truly can't buy the type of space you're creating and/or you have a sentimental attachment to the apartment/building. Good luck!

We are currently doing an extension right now, it is only around 400 sq ft and the total cost is around 400k all in.

There are many variables, for example the building next door was higher than 75 feet high so I was required to have 15 mil in insurance which costs an additional 20k. Things come up.

Lad -- thank you for sharing. I learned a lot from your post, and I'm exhausted just from reading it!!