Any project in Fairfax County exceeding 2,500 square feet in land disturbance will require a Site Plan prepared by a Civil Engineer and a Land Disturbance/Stormwater Permit upon County approval of the plan. This includes Tear Down & Rebuild, Home Additions, and Pool & Patio projects in Fairfax County. As land development continues on multiple properties across Fairfax County, stormwater management becomes an important factor that civil engineers must address and determine how to best manage stormwater runoff for each project.
The main purpose of an engineered Site/Grading Plan for projects exceeding 2,500 square feet is to protect the surrounding environment and develop land in a sustainable and responsible manner. One factor of this is to design how sediment will be kept on site inside the land disturbance area and prevented from washing downstream into the street and neighboring properties during construction. Another factor is to preserve mature trees over 12 inches in diameter and provide a certain required amount of canopy coverage on your lot. The biggest factor is keeping stormwater runoff equal to or less than the pre-development runoff rate. The Site Plan also provides important information for the contractor to follow and build the proposed improvements to the correct dimensions and elevations.
With the current trend of tearing down small, outdated houses to rebuild much larger houses, comes an increase in stormwater runoff. Multiply this across thousands of land development projects in Fairfax County, the increase in stormwater runoff adds up quickly and contributes to a much bigger problem to the environment. Therefore, there is increased concern and scrutiny to make sure new developments do not negatively impact downhill properties and streams.
As a method of addressing the rising concerns, all Residential Infill Lot Development Plans submitted in Fairfax County after September 5, 2022 must meet the new Localized Flooding Mitigation Policy. In an effort to standardize plan submissions and reduce lengthy county review times, the county has provided guidance for the Simple Infill Detention Method (SIDM).
Fairfax County resources for more information on the SWM Design Guidance:
- Fairfax County SWM Design for Residential Infill Lot Grading Plans
- Fairfax County Technical Bulletin # 22-06 on new 2023 SWM Policy
Stormwater Management for Projects Small and Large
Even if a project is just building a pool and patio, the same Stormwater Management rules apply as they would for a new house. In some cases, the pool and patio area is the same footprint size as a new house, essentially doubling the amount of impervious area on a lot. This significant increase in impervious area can have a major impact on the amount of stormwater to deal with. Some pool and patio projects, such as the one in the snippet below, can have just as many stormwater management requirements to meet as a new home, if not more. A tear down & rebuild project on the other hand may only have a minor net increase in impervious area when comparing old vs. new. The pool project in the below example is only adding impervious area and not removing any, which is why the stormwater detention system was very extensive due to the amount of water it had to detain.
From a stormwater regulation perspective, it doesn’t matter if the impervious area is from a house roof or a pool deck, they will have equivalent amounts of stormwater runoff. Therefore, whatever the net increase of impervious area is, we have to provide Water Quality (Treatment) and Water Quantity (Detention) through a limited number of stormwater options called Best Management Practices (BMPs).
Fairfax County Stormwater BMPs
With the new Fairfax County Simple Infill Detention Method (SIDM), we are limited to 4 BMP options with the following criteria:
1. Urban Bioretention Planter Box 1 (UPB 1)
- Up to five individual facilities can be designed for a single lot.
- Only roof area is permitted to drain to this facility type, with no more than 2,500 sq. ft. allowed to drain to an individual facility.
- This is the only facility type allowed to be located within 10 feet of a residential structure.
Example of an Urban Bioretention Planter Box Designed by DeMarr Engineering and Installed by DeMarr Construction.
2. Urban Bioretention Planter Box 2 (UPB 2)
- Up to three individual facilities can be designed for a single lot.
- Only onsite impervious area is permitted to drain to this facility type, with no more than 5,500 sq. ft. allowed to drain to an individual facility.
- This facility type must be located at least 10 feet from a residential structure.
Example of a detached Urban Bioretention Planter Box installed by DeMarr Construction.
3. Bioretention-Detention Rain Garden (RG)
- Up to three individual facilities can be designed for a single lot.
- Only onsite impervious + pervious area is permitted to drain to this facility type; with no more than 5,500 sq. ft., and pervious area at no more than double the actual impervious area, allowed to drain to an individual facility.
- This facility type must be located at least 10 feet from a residential structure.
Example of a Bioretention-Detention Rain Garden installed by DeMarr Construction.
4. Underground Pipe Detention (UPD)
- Up to two individual facilities can be designed for a single lot
- Only onsite impervious area is permitted to drain to this facility type, with no less than 600 sq. ft., nor more than 25,000 sq. ft. allowed to drain to an individual facility.
- Allowed pipe diameters are limited to 24-inch, 30-inch and 36-inch; and allowed materials are limited to HDPE, PP & CAP. A smooth pipe bottom is required.
- This facility type must be located at least 10 feet from a residential structure.
Example of an Underground Pipe Detention System designed by DeMarr Engineering for a Pool and Patio in McLean, Virginia.
When Do These Stormwater Management Requirements (SWM) Apply?
Any project with greater than 2,500 square feet land disturbance and a net increase in impervious area will trigger these SWM requirements to offset the increase in stormwater runoff. The only exemptions are “increases of less than 160 square feet of impervious area over existing conditions” on “lots exempted from the Stormwater Management Ordinance (SWMO).” Otherwise, the policy states: “Lots that are subject to the SWMO have no exempted increase in impervious areas. This policy applies to all such development, regardless of whether the disturbed area’s stormwater discharge will be concentrated or sheet flow.”
If you are planning to build a new home, addition, or pool in Northern Virginia or Washington, DC, please contact us for a quote on your next Land Development project.
