​An important function of the Mifflin County Planning Commission and Planning and Development Department is the review and approval of subdivision and land development plans, as authorized by the Pennsylvania Municipalities Planning Code (MPC), Act 247 as amended. Reviews and decisions are carried out according to the requirements of the Mifflin County Subdivision and Land Development Ordinance, adopted in 1995, updated in 2003, and amended in 2006.
                                        
The Mifflin County Planning Commission has the authority to approve, approve with conditions, or disapprove all subdivisions and land developments that occur in municipalities that do not have an ordinance. Those municipalities in Mifflin County include Bratton Township, Juniata Terrace Borough, Kistler Borough, McVeytown Borough, Newton Hamilton Borough, and Wayne Township.
 
In cases where municipalities have their own Subdivision and Land Development Ordinance, plans must be submitted to the County Planning Commission for review, and the Planning Commission provides comments to the municipality within 30 days. Municipalities in Mifflin County with an ordinance are Armagh Township, Brown Township, Burnham Borough, Decatur Township, Derry Township, Granville Township, Lewistown Borough, Menno Township, Oliver Township, and Union Township.