Our townhouse in downtown Manhattan was built in 1826, and was so messed up structurally over the years that landmarking it actually changed their mind about landmarking it when they found out.

Now we are thinking of adding on to back of the 2nd and 3rd floors and adding a complete new 4th story to it. The building has no additional load bearing capacity (it's only still standing by a miracle), and the only way to do this is to run 4 steel columns to create, in effect, a 3 story, 1700 foot, independent structure added on to the existing.

We plan to continue living in the house while the work is being done.

Has anybody done such a project?

How difficult was the job (the steel columns can be boxed in on the interior, and all of the other work will be on the outside of the existing structure)?

How horrible was the experience?

I haven't begun to price it. How much cost did the structural steel add to the job? (It will add to the value of the home because square footage here is very valuable.)

One reason for doing this is to add an elevator. Although they haven't been approved in NYC yet, we will leave room for one of the neat newish pneumatic elevators, that seem much better looking, smaller and cheaper:
http://www.vacuumelevators.com...
NYC is always at the rearguard of approving new technology, it seems.

We just discovered that the back of our ground floor and basement, concrete block from 1954, has no footing at all and sits on the soil line. I'm just grateful the soil hasn't frozen and heaved and the walls have not started to crack.

As usual, we have no idea what we are getting into, and we'd be most grateful for feedback from anyone who's done anything similar.

Here is a link that might be useful: Pneumatic vacuum elevators