Post Construction Requirements and Low Impact Development Resources

This page provides information and resources intended for development project applicants, architects, engineers, contractors, planners, landscape architects, municipal staff and the general public.

  • Development projects may be required to control pollutants in runoff from newly created or replaced impervious surfaces. Explore the Post Construction Requirements in Marin panel below for more information.
  • In Marin, some development projects must follow the BASMAA Post Construction Manual and must submit a Stormwater Control Plan. Explore the resources and templates in the How to Comply: Additional Tools and Guidance panel below.
  • Find out how to protect our creeks, bays and the ocean by exploring the Low Impact Development (LID) Resources panel below.
  • Post-Construction Stormwater Workshop presented by Dan Cloak. These training materials review how to incorporate Low Impact Development (LID) into north bay projects.
  • Slow the Flow, Keep Rain Onsite!
How to Comply: Additional Tools and Guidance

Certain development projects in Marin must follow the applicable guidance manual listed below:

Post Construction Requirements in Marin

Marin’s cities, towns and unincorporated areas must require designated development projects to comply with Provision E.12 of the State Water Resources Control Board (State Water Board) under the Phase II National Pollutant Discharge Elimination System (NPDES) Municipal Stormwater Permit (Phase II Permit). Provision E.12 requires site designs for new developments and redevelopments that are defined as Regulated Projects (or where required by the local agency) to minimize the area of new roofs and paving. Where feasible, pervious surfaces should be used instead of paving so that runoff can infiltrate to the underlying soil. Remaining runoff from impervious areas must be captured and used or treated using bioretention. Regulated Projects must also incorporate pollutant source control best management practices into the site design. Certain Small Projects and Single Family Home Projects (not part of a larger plan of development) must implement at least one site design measure and must show the decrease in runoff from the site.

Post Construction Alert – Fact Sheet on the E.12 requirements that are also described in the BASMAA Post Construction Manual.

Projects subject to the E.12 Requirements

The table below outlines the various categories of projects that are subject to E.12 Post Construction requirements.

Requirements at a glance from Table 1.1 in BASMAA Post Construction Manual:
Type of Project Project Requirements  Your Submittal
Single Family Homes: Projects that create or replace 2,500 square feet or more of impervious surface

Small Projects: Projects that create or replace between 2,500 and 5,000 square feet of impervious surface

Site Design measures:

1. Limit clearing, grading and soil compaction

2.Minimize impervious surfaces

3. Reduce runoff, for example by dispensing runoff to landscaping or using pervious pavements

4. Conserve natural areas of the site as much as possible consistent with local General Plan

5. Comply with stream setback ordinances/requirements

6. Protect slopes and channels against erosion

Follow the instructions in the template: Small and Single Family Home Project Stormwater Control Plan (Appendix C)
Required Projects: Projects other than single family homes that create or replace 5,000 square feet or more of impervious surface Site Design measures addition to those for small projects:

1. Route runoff to bioretention or other facilities sized and designed according to criteria in Chapter 4 BASMAA Post Construction Manual

2. Identify potential sources of pollutants and implement corresponding source control measures in Appendix A BASMAA Post Construction Manual

3. Provide ongoing maintenance of bioretention facilities

Follow the instructions in the template: Regulated Project Stormwater Control Plan (Appendix D)
Roads and Linear Utility Projects: Projects that create 5,000 square feet or more of newly constructed contiguous impervious surface Requirements vary Complete the “Model Sign-Off Form” (Appendix G). Contact your local stormwater coordinator regarding compliance options

For requirements at a glance for projects in San Anselmo see specific E.12 applicability table for that jurisdiction.

Preparing for requirements during construction

All post construction stormwater management requirements are separate from, and in addition to, requirements for erosion and sediment control  and other pollution prevention measures during construction. Construction projects over an acre are subject to the State of California’s General Construction Permit.

Low Impact Development (LID) Resources

If you’re planning to develop or redevelop a property, consider redesigning your drainage to reduce water quality impacts. Rain runoff carries pollutants to creeks and other water bodies. When rain flows over hard surfaces the speed and volume of water can cause creek erosion downstream. For ideas on how to use Low Impact Development design to protect fish and other wildlife in Marin’s creeks, explore the resources below and consider implementing the following practices:

  • Disconnect downspouts to let water run off your roof onto a splash block and into landscaped areas.
  • Install a rain barrel.
  • Slow down your roof runoff by connecting rain chains to your roof gutters.
  • Use mulch on plant beds to slow the flow in areas where water can seep in.
  • Consider installing a rain garden.
LID and Stormwater-Friendly Development Resources