Welcome or welcome back to my series of rants about the ridiculous metrics Montgomery County chooses to use when determining what permits, reviews and inspections to require for residential additions or new construction projects.

Today's topic: What Montgomery County gets wrong about DRAINAGE PLANS. As always, my suggestion for a fair and common sense law that applies equally to everyone AND actually directly address the issue is at the end.

There are more than a quarter million detached single family residential homes in Montgomery County. Every project and site is different, so I acknowledge that it is difficult to create legislation that makes sense for all projects and can be fairly applied across different Zones, property sizes, owners, builders etc. Unfortunately the way Montgomery County legislates drainage plan requirements just doesn't make sense.

MONTGOMERY COUNTY DRAINAGE PLAN BACKGROUND

When I say DRAINAGE or DRAINAGE PLAN I am referring to a requirement for an addition or new construction home to capture rainwater falling on the roof of the new structure. For detached single family residential projects this is usually accomplished with a dry well, though there are a few other methods that may be appropriate.

The current metric is that a drainage plan is required when;

adding 400sf or more of footprint on a lot that is fewer than 15,000sf. Detached Accessory Dwelling Units always require drainage, even if they are under 400 sf.

The theory behind this is that a larger lot is better able to retain stormwater on site, so engineered conveyance or capture methods are not required. This is not entirely wrong, but can also result in widely disparate project costs and requirements for the same size project depending on how much land the homeowners have. The same 500sf addition is treated the same if someone has 4,000sf of land or 14,000sf of land, and very differently if they have 16,000sf of land.

MONTGOMERY COUNTY DRAINAGE PLAN QUIZ

Read the following scenarios and view the following images. Can you determine which projects require drainage plans and which don't?

(Hint: if you only consider which projects are larger, have more roof area and will contribute MORE to uncontrolled runoff you will fail the test)

QUESTION #1

Which of the following two accessory structures requires a drainage plan to capture rain?

Both houses:

• are on a 7,500 sf lot
• have a 350sf footprint
• have 425sf of roof area
• have a CMU foundation
• have fiber cement siding
• have plumbing service
• have electrical service
• have mechanical service

A

B

ANSWER #1

Structure A requires a drainage plan because it has an oven, technically making it an Accessory Dwelling Unit. ADUs require drainage regardless of size. I wonder how the rain knows not to cause flooding before it falls on Structure B?

QUESTION #2

Which of the following two additions will require a drainage plan?

PROJECT A

• Existing 3,600 sf house
• Existing 1,800 sf lot coverage
• Adding 1,000 sf addition
• Existing 2,000 sf of roof area
• Adding 1,200 sf of roof area
• Addition is 16 feet from adjacent property line

PROJECT A

• Existing 800 sf house
• Existing 800 sf lot coverage
• Adding 400 sf addition
• Existing 925 sf of roof area
• Adding 625 sf of roof area
• Addition is 24 feet from adjacent property line

ANSWER #2

Project B requires the drainage plan because their lot us under 15,000 sf and the Project A lot is over 15,000.

Project A has double the roof area of Project B and more than double the impervious area. The addition roof is pitched so that all the water will run off toward the Project B property. Yet because Project A has a little bit more land area, thus they do not require any drainage plan.

A DRAINAGE REQUIREMENT THAT MAKES SENSE

ALL ADDITIONS OR NEW CONSTRUCTION HOMES SHOULD BE REQUIRED TO PROVIDE DRAINAGE FOR ALL LOT COVERAGE AREA BEYOND XX%.

I don't know what number to use for that XX. I'll leave that up to Civil Engineers and other land use experts. But there is all sorts of precedent for this. Washington DC zoning restrictions include a required Green Area Ratio. It varies from zone to zone, but might be 60% in a residential zone. This is a different requirement from, and applies in addition to, Lot Coverage, which usually restricts the footprint of a house (size as viewed from above). A Green Area Ratio of 60 means that 60% of a lot must remain as undisturbed soil…no house, gazebo, deck, driveway, areaway etc.

Without adding an explicit Green Area Ratio to Montgomery County Zoning we could use a lower Lot Coverage ceiling to trigger a drainage requirement. This would fairly apply to all projects in all zones and result in more stormwater being captured from projects that are actually larger.

For example: Typical Lot Coverage in Montgomery County R-60 is 35% (it may be lower in some instances, zoning can get complicated!) Any addition should still be allowed to build up to that limit, but must provide drainage for all coverage beyond 20%.

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