You don’t see a lot of properties like 109–113 East 90th Street on the market. You might see a carriage house, and you might find a firehouse, but rarely are they linked in such a way, with a courtyard, just off Park Ave. The firehouse currently houses an art gallery and offices, which boast fourteen-foot ceilings with original pressed tin and hardwood floors that date back to the nineteenth century. And perhaps the biggest selling point? It’s not landmarked, so you can tweak the features as you see fit — even building something sunny and spectacular on the as-yet-unfinished roof.