Chapter 18.28 OPEN SPACE DISTRICT – OS DISTRICT This chapter is included in your selections. Sections: 18.28.010 Intent. 18.28.020 Permitted and principal uses and structures. 18.28.030 Conditional uses. 18.28.040 Minimum lot size. 18.28.050 Minimum setback requirements. 18.28.060 Maximum height of structures.