Adding Floors to building

Get some professional help on this one -- a good architect and code consultant supplemented by a structural engineer and perhaps a contractor or two. They will need to assess the actual existing building, consider any code updates in the rest of the structure that may be required in conjunction with the Alt-1 application, etc. If you are prepared to spend $1MM and up on expansion after purchase, don't hold back on spending $3000-$5000 in professional services to be confident in the purchase and budget.

Give a call to All Renovation Construction, they are very well experienced in Manhattan in regards to townhouses and brownstone (expansions). You can reach them at 718-351-4099 or via their website at www.allrenovationllc.com

Most of the time, you cannot simply just add floors to the top of a building like Lego blocks; the entire building must be fortified to hold the considerable extra weight. For most buildings, the expense of the wholesale reconstruction is often not worth it.

An interesting subject , and worth the cost

Check out StreetEasy's blog Own Your Home for an overview of the ins and outs of flooring your apartment. A local contractor gives simple tips and practical advice!
http://ownyourhome.streeteasy.com/flooring-your-home-made-easy/