Do we need a building permit?

We own a 4-story/4-family brownstone in Boerum Hill and currently live in the garden level apartment, with three tenants in the three remaining units. The parlor level tenant is moving out and we plan to join the garden and parlor levels into one apartment by building a small wall to privatize the existing stairs between the two levels. We also plan to remove three interior walls on the garden level. This would involve some electrical rerouting and moving one sink about three feet. We’ve had a certified structural engineer and two architects look at exposed joists in key areas. The engineer did the calculations and it is agreed that the plan to remove the interior walls is structurally sound.

We are considering doing the work without a building permit to avoid the inevitable time, hassle and cost of getting the city involved. We are very capable and responsible DIYers, so it’s not a question of safety; we’ll do it right. The question is: what is the risk of not filing the work and getting a permit, and is it worth it to avoid it?

If the permit is the thing to do we’ll do it. But we’d love to hear honest opinions and stories from all sides.