A Planned Unit Development may be developed in stages if the following standards are met:
(a) The location and approximate time of construction of each stage are clearly marked on the development plan.
(b) At least fifteen (15) percent of the dwelling units in the development plan are included in the first stage.
(c) At least fifty (50) percent of the dwelling units in any stage shall be completed before any commercial development shown in that stage shall be completed.
(d) The second and subsequent stages are completed consistent with the development plan and are of such size and location that they constitute economically sound units of development. In no event shall such stages contain less than fifteen (15) percent of the dwelling units included in the development plan.
(e) To encourage flexibility of housing density, design, and type in accord with the purposes of this chapter, gross residential density may be varied from stage to stage. A gross residential density in one stage which exceeds the permitted average gross residential density for the entire Planned Unit Development must be offset by a gross residential density in a subsequent stage which is less than the permitted average gross residential density for the entire Planned Unit Development.