The impact of those accumulated tax breaks is poorly understood, said Harold Shultz, senior fellow at the Citizens Housing & Planning Council. Yet while lawmakers regularly tinker with program terms when laws come up for renewal, he said, there is little practical prospect of ever redrafting the entire system to ensure that it is as effective as possible. “It would be hard to rewrite the thing from the top,” Shultz said. “Here’s the thing to remember: People are still coming to New York City. They need a place to live. We have a very tight private housing market. We need for the private housing market to build housing.”