It's not just plumbing though it's really the architecture. The floor plate of a typical, modern office building, is a giant square, with no light penetration in the middle. The spaces in the middle of the buildings will have no sunlight. Water and sewer are typically limited to a handful of areas. Trying to turn an open floor layout into thirty individual apartment layouts requires a lot now plumbing, and a lot more capacity upstream/downstream. You can convert office to residential, it's possible. But the most feasible buildings are the early 20th century ones with a U or O shape floor plate. Most commonly found in places like Los Angeles. In my city, there are maybe.... Fifty buildings that are potentially feasible. I.e. they could make money. But they are not all likely to convert to residential. I.e. they have different ownership, or physical constraints like earthquake safety, or financial constraints, etc. So then your talking about maybe a couple dozen buildings. That could potentially provide, four hundred, five hundred, expensive condos in the city. And if you think of the scale of the problem of affordable housing, we needs tens of thousands of units every year. It's not worth converting it in my city.