If David's architect advises that the house must be within 20 feet from the lot line while the zoning ordinance requires a 30-ft setback, what must David apply for?

In this scenario, the correct step for David to take is to apply for a variance. A variance is a legal exception to a zoning ordinance, allowing the property owner to deviate from specific restrictions such as setback requirements. When the guidelines established by a local zoning ordinance—like the 30-foot setback in this case—are stricter than what an architect recommends or what a property owner desires, applying for a variance is necessary to gain the approval for that deviation.

David's architect's recommendation to have the house within a 20-foot setback indicates that the proposed placement does not comply with the established zoning law. Thus, to legally position his house closer to the lot line than what is permitted, David needs to seek this variance from the local zoning board, which will evaluate the request and determine if there’s a valid reason to allow the exception based on the unique circumstances of his situation.

Other options such as a special permit or a waiver do not directly address the need to alter zoning restrictions in the same way that a variance does. A development agreement typically involves broader conditions for development and does not specifically pertain to changing set-back regulations. Hence, in David's case, the solution lies accurately within the application for a variance.