Polk County, IA Zoning Overlays & Bonuses: Density Bonus Law (2026)
Key Facts
- Density bonus
- 5% more units, rounded down
- Minimum development size
- 20 units
- Runoff reduction required
- 50% or greater, engineer-certified
- Development form required
- Cluster or Planned Development only
- Buffer strip minimum width
- 50 feet
- Trail minimum width
- 6 feet
Summary
Unincorporated Polk County awards a 5% density bonus, minimum 20-unit development, to Conservation Design subdivisions whose engineer proves added stormwater controls like bioswales, buffer strips and native grasses cut runoff by 50% or more from every proposed structure and graded area. The bonus rounds down to the nearest whole unit.
Section 3. Density Bonus
Density bonuses may be provided by the County for developments meeting purpose and intent of this conservation ordinance and when additional on-site storm water management measures and practices are incorporated into the development. The additional units should not change the character of the proposed development or adjacent developments.
(A) To obtain a density bonus, the developer's engineer must demonstrate that the additional on-site storm water management measures, including but not limited to retention areas, detention areas, buffer/filter strips, bioswales, bioretention cells, and native grasses will reduce storm water flows generated from all proposed structures including all proposed graded and fill areas by 50% or greater. Calculations must show storm water runoff rates of development with the new storm water management measures and without them.
(1) ... Density bonuses are awarded if the previous standard can be met as follows: A five (5) percent increase in the number of units with a minimum development of twenty (20) units. The bonus is rounded down to nearest whole unit.
Official source re-checked September 7, 2026: the cited page had not changed since it was quoted.
Full Breakdown
Article 9 lets developers choose a Conservation Design option built as a Cluster or Planned Development, aimed at protecting water quality, managing on-site stormwater, preserving farmland and open space, and cutting impervious surfaces. Division 2's Low Impact Development options include native grasses and plants in buffer/filter strips, native prairie establishment, bioswales, bioretention cells and rain gardens, plus permeable paving if the county approves it at site development or preliminary plat. Division 3, Section 3 sets the density bonus: the developer's engineer must show, with runoff-rate calculations comparing the site with and without the added stormwater measures, that retention areas, detention areas, buffer/filter strips, bioswales, bioretention cells and native grasses cut stormwater flows from all proposed structures and graded or filled areas by 50% or more, without changing the character of the development or its neighbors.
Meeting that standard earns a 5% increase in unit count, but only on developments of at least 20 units, with the bonus rounded down to the nearest whole unit and added to the county's Density Calculations Worksheet total. Section 4 requires the resulting open space to be permanently restricted from future development, owned by a homeowners association or county-approved entity, with buffer strips at least 50 feet wide and trails at least 6 feet wide, and Section 4(F) allows on-site septic in open space if sanitary sewer isn't within 300 feet, with an easement reserved for a replacement system.
Violations & Fines
Conservation Design and its density bonus are administered through the standard zoning permit process; a development built or occupied at a density above what the ordinance and its worksheet allow is treated as a zoning violation under Article 23, drawing a Zoning Administrator notice, a correction deadline, a possible stop-work order, and civil or criminal remedies if uncorrected, with each day of continued violation counted separately.
Frequently Asked Questions
How does a developer earn a density bonus in unincorporated Polk County?
How big does the bonus make a development?
Who owns the open space created by a Conservation Design bonus?
Sources & Official References
Other rules in Polk County
Compare Polk County to another location·View the Iowa zoning overlays & bonuses overview
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