We frequently need to (file and/or) amend 421a tax applications of our Developer clients at Department of Housing Preservation and Development (HPD) as the development project progresses.
Usually a change in number of dwelling units or unit area, or break up of units (was one-bedroom, now is a studio) requires this. Change in commercial square footage or number of stores would also trigger this requirement.
Constructing a building is a long project, let's say with an average life span of 3 years. Therefore the changes causing an application amendment will happen, due to change in financing or change in market conditions.